John Barth writhes on the edge of his seat, shuttling a tanned lank ankle from floor to knee, swiveling his torso like a Jhoon Rhee dropout. All this incessant clamor for reality!

The man has every right to be relaxed. Outside, the ancient swell of his Chesapeake Bay wraps the cove in a deep pewter sheen, and his trim sloop lolls in its moorings. Across the lawn, his sprightly wife is gardening. And across the country, his new seagoing novel "Sabbatical" is blowing up a critical storm.

Yet here in his snug, bayside house, Barth is in full sedentary swivet, fending off questions of personal fact. "I'm a fidgety man," he says, hugging a knee, plainly unhappy at talking about himself except in the third person, deflecting inquiry by quoting other writers in the marshy vowels of his waterman's drawl. The intractable banality of mere biography--who needs it?

But get him on an idea and suddenly this most cerebral of novelists slumps in comfort. The lights come up on the mental midway, the old intellectual Tilt-A-Whirl slowly begins to spin, and pretty soon he's lost in a fun house of interlocking concepts.

For example, the origin of "Sabbatical," which floated up in a pure Barthian welter of abstraction. He was commissioned to write a screenplay for WNET, the New York public TV station, and "I was preoccupied with the theme of sperm and ova borrowed from an early story." In that piece, "Night Sea Journey" (1968), a melancholy spermatozoon broods on his mission: "My trouble is, I lack conviction. Many accounts of our situation seem plausible to me--where and what we are, why we swim and whither." Ironically, it had been written "to exorcise a pattern that had been riding my imagination for too long: the cyclical adventures of traditional mythical heroes wandering," the "funniest version" of which was the odyssey of egg and sperm.

But "I wanted to write a sequel in which the two principal characters were going to be ova--human actresses, of course, sort of in inflatable costumes--coming downstream while a spermatazoon left over from 'Night Sea Journey' would be swimming upstream." Barth is not laughing. "And the three-act play would have to do with the confluence of these two trajectories, as well as the differences between the two women. The play turned out to be non-producible. They wanted to do it, but it was a complicated production. It all happens in the water--swimming in wet suits with tails and so forth--and the actresses in these inflatable envelopes, navigating, paddling." After rejecting a dry-stage version ("much too artsy," says Barth), WNET concluded that the aquatic effects would be prohibitively expensive. "I see their point. It was the loveliest rejection I've ever received."

He was left with the notion of twin stories: The one above and "a realistic prose narrative of human lovers sailing upwards, who may or may not be the products--or producers--of those sperm and eggs. Thus the play became the scaffolding for the construction of the novel." Later, "it seemed to the unanimous tribunal of my wife, my agent and my editor--and ultimately myself--that the scaffolding should be removed."

The result is one of his most accessible--and most unexpected--books. After seven other volumes and 30 years of teaching, 10 of them at Johns Hopkins, "the idea was to take a kind of a breather," he says. "It's an unusual leopard who changes his spots in the arts, but you do check over your spots now and then, rearrange them a little bit."

A spot check reveals that from "The Floating Opera" and "The End of the Road" in the '50s, through "The Sot-Weed Factor" and "Giles Goat-Boy" in the '60s, to "Chimera" and lately "Letters" in the '70s, Barth has earned a madcap eminence (and occasional odium) for huge and bawdy intellectual fables, philosophical vaudeville, rococo parodies of antique literary forms. They are self-conscious tales--telling the reader he is reading a story telling itself--whimsically bedecked with literary allusions, inkhorn diction, mythic figures made homely and plain folks made mythic.

But in "Sabbatical," he says, "I have resumed a romance with realism." And at first, it even seems autobiographical: Writer Fenn Turner, 50, and his second wife Prof. Susan Seckler, 35, take a sailing vacation. (Barth, 52, and his second wife Shelly, 37, a high-school literature teacher, are devoted sailors.) The fictional pair cavort in 33 feet of sleek teak. (Barth has 25 feet of fiberglass: "One of the purposes of art is to give you boats you can't afford.") Each is a twin; Barth has a twin sister.

They visit Fenn's aging parents on the Bay (Barth was born in Cambridge, Md.). And their argosy obliges them to choose among diverging life-options--including whether to have a child (Fenn has a son and new grandson; Barth, father of three, is a recent grandsire) and how to engage the moral issues of American society. (Barth, often criticized for blithe disregard of "real life," fires political salvos aplenty.)

However, "I don't write autobiography," Barth protests, squirming again. And the book soon reveals itself to be a multiplex meditation: on the union of opposites ("a good marriage and a full-rigged sailboat," he says, "embody harmonious tensions between contraries," as do Susan and Fenn, pragmatic and romantic, etc.); on the symbiosis of life and art ("The doing and the telling, our writing and our loving--they're twins"); on the metaphorical parallels among sea journeys, heroic quests, the physical vectors of conception and the ineluctably mythic shape of every human life. All served up in a richly alliterative prose, festooned with footnotes and told in three voices: Susan's, Fenn's and a collective we. "Coming together to make a thing which is at once both of them and neither of them," says Barth. The Contaminated Novel

These ideas were afloat, but the book didn't emerge until 1978, when the very un-abstract carcass of ex-CIA official John Paisley, dead by mysterious circumstances, bobbed up in "my home waters and the novel certainly was occasioned by that case." This literal intrusion of the real world becomes a ditto in the novel: Barth simply reprints 20 pages of Paisley stories from The Baltimore Sun. "If you're gonna pollute a work of fiction with an enormous gobbet of fact, you might as well lay it on. That's not unprecedented--the novel is the most hospitable genre to any kind of contamination."

Even a belated sense of duty from an academic writer: "I'm a simple-minded liberal who's indignant about the destruction of Chilean democracy and so forth and I don't believe that our hands are perfectly clean." Although an erstwhile anti-war activist and avid reader of engage' nonfiction, politics is "something that's been properly absent from my fiction. On the other hand, I'm certainly not a man without opinions or reactions to things. And I don't believe, as some of the great high modernists do, that that sort of thing has no place in fiction. I believe that the muses will smile or not smile whatever end of the spectrum a writer comes from."

Whatever end he's working at the moment, Barth is never far from his native Eastern Shore. Maps of the area cover the walls, even the plastic placemats, here on Langford Bay where Barth wriggles in recollection.

The second son of a candy-store owner, restaurateur and judge in orphan's court, Barth grew up in "the boundless tidal marshes" of Dorchester County. In that landscape, he writes, "where horizontality is so ubiquitous that anything vertical--a day beacon, a dead loblolly pine--is ipso facto interesting, the abstract wish to distinguish oneself somehow, anyhow, seems pardonable to me." Pardonable, too, an early yearning to link one's life with the larger archetypes: "One of the things that fascinates everybody about tides," Barth says now, "is that any little creek connects with the waters of the world. In the same way, our very homely, far-from-heroic personal experiences--simply because they are human experiences--contain the general pattern and connect with the great myths."

Jack was born with a twin sister Jill, which resulted in an early sense that "language is for relating to others" (for twins, "nearly everything went without saying"), a premonition of the power of the incest theme ("natural if you've grown up hearing the bawdy versions of the nursery rhymes since you were 6") and a personality "somewhat introverted, somewhat shy. One occupational hazard of being a twin is that one grows up solitary and self-sufficient." Still, he played the drums in a local jazz group--"rhythm seems to come more naturally to me than melody and harmony"--and briefly attended the Juilliard School, determined to be a jazz orchestrator. The message outlasted the medium: "At heart, I'm an arranger still, whose chiefest literary pleasure is to take a received melody . . . and, improvising like a jazzman within its constraints, reorchestrate it to present purpose."

He took a scholarship to Johns Hopkins, years behind his better-educated classmates: "They had heard already about the Renaissance, the Enlightenment and the rest; I was lost in the Dark Ages. They were discussing the architecture; I was trying to find the men's room. Everything was news." (No wonder later books would flaunt their author's erudition: "Giles Goat-Boy," he feels now, "rubs the reader's nose in a sophomoric allegory.") But he rapidly closed the gap, both in life and letters: The undergraduate influx of returning World War II veterans resulted in "a boom in early marriages among those of us who had not been to war. I married when I was 19." Literary role models proved equally persuasive, but he gradually shook off the influence of Joyce, Faulkner and "The Arabian Nights." In 1952, he took up teaching by "a kind of passionate default," first at Penn State, later at the State University of New York at Buffalo, and "by the time I was 25, I'd fathered three children and written two novels."

But in the late '50s, "I realized that realism was tying my hands," and he returned to the 18th-century origins of the novel for "The Sot-Weed Factor" and found "more elbow-room." And more room for self-conscious narration: "When I was a student in Baltimore, I used to love to go down to the old Hippodrome Theater where they still had live vaudeville between the movies. My favorite kind of acts were the magicians, tumblers and acrobats," and he preferred the kind "who is always talking about what he's doing while he's doing it." He began writing the same way, most critics raved, and by the '60s it was a trademark technique. Old Flames

He would find two long loves in the same decade. Divorced from his first wife, in 1969 he met a former student from Penn State, Shelly Rosenberg, while he was giving a reading in Boston. They were married in 1970--about the same time Barth was re-romancing his old flame Scheherazade. Although it's "famously true that we come away with real tears in our eyes from a third- or fourth-rate film," he says, "I don't recall ever weeping actual tears on the printed page." But to this day, "I still can't hear the violin or the oboe motif in Rimsky-Korsakov's 'Scheherazade' without a chill." They first met in the library stacks at Hopkins, and she became his mentor and metaphor. "What for Scheherazade is literally the case is figuratively the case for writers, since in some sense we're dead if we don't please the hearer--and for all the rest of us, since we spend much of our lives telling each other stories. That's a more distinctive human characteristic than language itself, since all sorts of other things chatter at each other."

She became the subject of one of the three interrelated novellas titled "Chimera," which won Barth the National Book Award in 1973--the same year that an offer from Johns Hopkins catapulted him back to his ancestral fens. Since then, he has lived quietly between Baltimore and the bay, in the deep penumbra of the literary limelight. "So much do I like staying home and writing, and because I am not an aggressively gregarious fellow, I suspect that were it not for teaching, my contacts with other kinds of people would be more limited." In New York, he says, "I would quickly suffer from sensory overload," although "once a year my wife and I usually go up to the Institute of Arts and Letters ceremonial, which really is like a kind of living Madame Tussaud's wax museum. I'm as much of a rubbernecker as anybody in the hall, saying things like, 'Look's there's Sol Steinberg!' "

At home he reads Nabokov, Borges, Gabriel Garcia Ma'rquez--"the second-generation masters of modernism"--along with Italo Calvino, Gu nter Grass and a gaggle of Americans: Updike ("a remarkable artist"), Cheever and "most of those people whose names are often lumped with mine." The group includes Donald Barthelme, John Hawkes, William Gass, Robert Coover, Ishmael Reed--"all the ones Gore Vidal doesn't like. As Barthelme says, I like the team they put me on." And when not reading, he can be found solitarily flogging the paradox he posed years ago: Socrates found the unexamined life not worth living, but Oedipus discovered the examined life to be unlivable.

The same might be said of literary reputations. Barth has accumulated a boatload of laurels; has been the subject of myriad PhD theses ("it gives you a sort of posthumous feeling before the fact"); even has a fan club, the Society for the Celebration of Barthomania, which claims a number of nationwide chapters and offers a choice of three bumper stickers, including "John Barth is God." "God bless them," says Barth. They call every year to sing him "Happy Birthday," "they send me the minutes of their meetings, which are hilarious," and "one reason I stay away from them is that I would come across as such a slow-footed clod."

However, "Fortunately for the size of one's ego, there are always at least as many critics telling you to go back to marsh and stick your head in it." They've been doing it for years, but with redoubled zeal since 1978 and the publication of John Gardner's watershed screed, "On Moral Fiction." The swan of Batavia castigated Barth for verbal game-playing and self-conscious cerebral dither. Barth is tired of "that old argument. If stories talk in an entertaining and even exciting way about themselves while they're talking about the motions of the human spirit, the passions of the heart, then they're accomplishing a kind of moral purpose if by 'moral' you mean they're an illuminating reflection of our experience of being living human beings." Pages of Pasta?

But the same charges haunt "Sabbatical." Although The Washington Post and The New York Times have been positive, Time magazine calls the characters "monomaniacal monsters," their story "smug, self-deluded solipsism." And The New York Review of Books, with inordinate dyspepsia, hangs Barth and John Hawkes ("masters of metafiction") on the same gibbet: "Elaborate nothings are what these novels are, curling heaps of phenomenological macaroni."

"The ferocity!" Barth marvels. "My skin is fairly thick, but this almost personal animus, this hair-trigger ill-will . . ." Best to take the long view: "That kind of sport or play in the medium goes out of fashion . . . readers and critics get tired. But it's as old as art itself--our first operas are about opera, our first novels are about documents." And new as the news: "Every night we see John Chancellor come on, and the first shot you see is a shot of the cameras taking a picture of John Chancellor watching a monitor on which there's a picture of the camera taking the picture."

But the critics "bother you only until you get back to work," says Barth, who compares himself to an auto designer: The one that's in the showroom now is the one he designed three years ago. "While that hurts, it doesn't hurt your production, because that's not where you are any more."

Where's that? Well, there's the next book, a "downstream complement to 'Sabbatical.' Like a hack songwriter, I have a title: 'The Tidewater Tales: A Novel.' " But first, "I'll take a different kind of sabbatical for a few months by writing a couple of essays to clear my head." Like this: "I've just cracked the riddle of Scheherazade's menstrual cycle! Nobody in the country recognized that there was a problem there." It seems likely. "The question I asked myself is, Why are there 1,001 nights instead of 202 or 5,497?" Readers will have to wait for the essay, but the clue is in the three sons Scheherazade suddenly brings in at the end of the tale. "I looked at it a little more closely, with the help of a pocket calculator and a manual of gynecology and obstetrics, and suddenly understood everything!"

This way to the funhouse! The lights are blazing on the midway, the old Tilt-A-Whirl is moving. And John Barth is riding, riding.