Retrospective exhibitions often come too late or too early in an artist's career -- posthumously, or when the subject is barely past artistic puberty.

Robert Arneson's Hirshhorn retrospective, the best contemporary one-man show the museum has had, is perfectly timed for two reasons: His achievement, at 55, is much larger than most people realize, and he's still going full tilt, exploring territory and taking risks. The most recent works will surprise even those who have followed his career.

Arneson also is the perfect subject: His giant ceramic busts and sprawling floorpieces -- silly, satirical or profound -- often relate to his struggle as an artist, and thus chart not only his life and accomplishment, but also the generic nature of a life in art.

Starting with his early efforts to find his own medium, his own creative voice and, ultimately, to make a distinctive statement that finds acceptance, Arneson's show traces every artist's odyssey and dream. It also carries the hopeful message that creative juices can keep flowing, no matter what the odds.

Anyone who thinks you can't get serious about an irreverent funnyman like Arneson should take another look.

In the easygoing chronological installation, Arneson's 1966 ceramic "Typewriter" with polished fingernails for keys is the most familiar of the clay objects from the '60s, and it reflects the influences swirling around him as he turned irrevocably to clay sculpture as his chosen medium around age 30.

"Pop Art" was ascendant in New York at the time, but there was also a revolution under way in the San Francisco Bay Area (where Arneson was born and still lives), led by ceramic artist Peter Voulkos, who insisted that clay should be used, as it was in the past, for sculptural, expressive ends, not just for pots. Voulkos' ideas launched a genre swiftly adopted by many California artists, with Arneson chief among them. From the first, however, Arneson added his own humorous, Dada-inspired twist by making "Pop"-object sculptures such as toilets, and toasters with protruding fingers.

With the stimulation of fellow artist-teachers at the University of California, Davis, (including Roy De Forest, Wayne Thiebaud and William Wiley, among others) Arneson continued to make what became known as "Funk Art," characterized by a peculiarly sophomoric brand of humor that fit the '60s. It was not until the '70s, when Arneson began making portraits of himself along with portraits of other artists he admired, that his own imagination and originality came to the fore.

These variations on grand-manner marble busts, all perched on columns or pedestals, are a recurring theme throughout the show, and a measure of Arneson's changing moods. He is pure comic audacity at first as he merges himself with a column in "Classical Exposure," his bearded, cigar-smoking head on top, and his feet and other bits of anatomy naughtily peeking out the bottom. He is elsewhere an aviator, a kiln man and in his "Balancing Act," a hard-working juggler of bricks.

"Art isn't easy," these works seem to say. Signs of struggle and hard work are everywhere, not only in the grimacing faces, but also in the complexity of huge floor-pieces like "Casualty in the Art Realm," in which he deals with the artist's plight, or better still "Fragment of Western Civilization," in which his head is a bit of sculptural debris among the ruins.

Also from this period are the spoofs on other artists that include a highpoint of the show, the wonderful "Pablo Ruiz With Itch," a bust of the young Picasso atop a column, his face rendered in the early Cubist style of his famous painting, "Les Demoiselles D'Avignon." The artist is shown trying to scratch his own back, something famous artists often do. Picasso's pose also mimicks the pose of one of the nude prostitutes in "Demoiselles."

Almost as unforgettable is "George and Mona in the Baths of Coloma," a leering George Washington and elusive Mona Lisa bathing together, a hilarious juxtaposition of two icons, one representing art, the other, the mighty dollar.

In the early '80s, worn down by a long bout with cancer, the rejection of his commissioned portrait of slain San Francisco mayor George Moscone, and domestic upheaval, Arneson's portraits -- and other works -- take a decidedly different turn. He appears resigned and almost contrite in "Portrait of the Artist as a Clever Old Dog," and is suddenly gripped by a desire to chronicle the facts of his own life in "Self-Portrait With Bio-Base," a noble, classical head not in his beloved clay but in eternal bronze. The bust is set on a column very like the Moscone column that was found so objectionable, and is similarly covered with graffiti-like notations of family birth dates, marriage and divorce dates and a drawing of himself as Leonardo da Vinci's perfect man, no doubt an ironic reference to his health.

It is a poignant piece, more poignant the more you look at it. And so is this show. With giddiness gone grim, he has since moved past the matter of his own, now healthier life (and in the case of "Ass to Ash," his death) to deal with the larger issues of the life and death of the world as it faces the nuclear threat. Though the giant, bashed-in cranium of a child in "Holy War Head" is a powerful image, most of the rest of these antiwar pieces are simply too cliched to be affecting. As for his "General Nuke," with a bomb for a nose, and the other drawings of grotesquely masked military leaders, they are too angry and screechy to be anything more than cartoons.

The new seriousness in other realms, however, such as the later self-portraits and the mesmerizing head of Jackson Pollock that ends the show, reflects a new depth that can only enrich Arneson's future work.

It is interesting to note that a good deal of Arneson's earlier work has mellowed with age, and that even pieces that offended at first encounter now seem tamed by the passage of time. The Moscone sculpture may also seem tame one day, though at present it seems raw and a bit too frank, dealing as it does with a still open wound.

The whole show, in fact, leaves Arneson looking better than ever, and makes us realize how selective curator Neal Benezra has been in putting this retrospective together. He has appropriately left out Arneson's many excesses, focusing on works that have depth and meaning.

The exhibition was conceived when Hirshhorn director James Demetrion and Benezra worked together at the Des Moines Art Center, where this show was organized and first shown. It augurs well for one-person, contemporary shows at the Hirshhorn, which has had an undistinguished record in such exhibitions.

A fine catalogue with an unusually intimate and revealing essay by Benezra has been published and deserves to be read. The show will travel to the Portland Art Museum, Oregon, after it closes here on July 6.