Summer television is a stale cookie.

In summertime, viewing levels in the average TV home drop by as much as two hours per day from wintertime highs, according to the A.C. Nielsen Co., and the number of TV-equipped homes actually watching television during peak prime-time hours drops from around 48 million to about 37 million.

Obviously the networks can't be bothered whipping up exotic new creations for only 37 million families. So they maximize the profit margin and minimize effort by clutering the airwaves with a species as common to summer a mosquitos. Reruns.

This summer will be anything but a revolution in that regard. Most shows have been into repeats for several weeks already. The "new" offerings tend to involvle half-fresh faces in old formulas; the Starland Vocal Band, Marilyn McCoo and Billy Davis Jr., mimes Shields and Yarnell starr in musical-variety turns on CBS; NBC has an abbreviated situation comedy called "The Kallkaks"; ABC is shifting some of its hit shows around to different time slots in the hope perhaps that viewers won't notice they're getting recycled goods.

In the early days of television, there was a greater likelihood of experimentation and innovation during the summer months than there is now. Television wasn't the biggest business in America then, and even though television has always made money in the summer (television always makes money, period), it tended not to be hot competitive territory.

That meant that an idea considered too riskily unusual for regular season exposure might get a trial run in the summer. A program now regarded as a TV classic, "Mr. Peepers," starring wally Cox, went on the air as a summer replacement show on July 3, 1952. It stayed on until 1955.

Ed Sullivan't "Toast of the Town" signed of for the first time June 20, 1948, during the very dawn of television,and didn't sign off until 1971. "I Remember Mama," one of many programs to be successfully translated from radio to TV, first aired on July 1, 1949.

"The $64,000 Question," which became first a smash hit and then the touchstone for a sordid era all its own, premiered on June 7, 1955. And Paddy, Chayefsky's "Marty," probably the best play ever written for television, just barely missed falling into the doldrum days when first shown on May 24, 1953.

Of course the good shows were exceptions. Generally the summer was filled with reruns and with scaled-down musical-variety shows starring more or less proverbial pinck hitters. Britain's Dave King regularly filled in for NBC's Perry Como. John Raitt and Janet Blair teamed to replace Dinah Short. Pat Boone was a bigger star in the late '50s than Andy WIlliams, so Williams was Boone's summertime-stand-in.

With an inevitability both grim and giddy, perennial summer replacements like "Pantomime Quiz" and Masquerade Party" popped up year after year with the pollen. The summer of 1956 even saw a very brief television career for "Ina Ray Hutton and Her All-Girl Orchestra."

No, it wasn't the Renaissance.

Yet here and there were exceedingly bright spots. In 1962 and 1963, NBC alloted weekly half hour each summer for director musical show called "The Lively Ones." Shear played with the TV medium, hauling cast and crew to such locations as Washington D.C., where Vic Damone, the host, sang at the Tidal Basin and guest Benny Goodman played on the Capitol Steps.

In the studio, George Gershwin was a guest one week when Shear installed a Gershwin piano roll in an old upright and shot it as it rinky-tinked through the open window of a synthetic tenament. "Lively Ones" was the television equivalent of ice-cold lemonade. It proved that summer TV, like summer reading, could be satisfying if light.

In 1968, CBS dared to devote an hour a week during the summer months to "The Prisoner," an absorbing, daring, not altogether coherent quasi-sci-fi series starring Patrick McGoohan as "Number Six," a Kafkasesque victim of an unidentified Adding Machine world.

The look and the sound of the show, filmed in black-and-white were harmoniously stylized; you couldn't confuse it with anything else on the air. It was demanding viewiwng that could not be picked up and dropped at random the way most TV shows can - just the kind of innovative thing networks should underwrite, at least during a period when ratings are less a matter of life and death than usual.

"The Prisoner" because a cult object but the only cult TV networks are interested in is the cult of everybody. That holds true now, even in summer. Time not filled with reruns is often given over to unsold old pilots for series that never made it to the schedule. Network money helped produce many of them, so this is a way the networks can get back a few bucks and use viewers as guinew pigs.

You don't find innovation in these shows, generally, because they were made with the regular season in mind - thus, written according to formula by committees or computers. Exceptions are rare.

And so for the American television viewer, summer means little more than the certain recurrence of three discouraging words that have become virtual annual ritual: "There's nothing on."

Why does there have to be a "summer" TV season at all? Like other things, television inherited the idea of a laid-back summer from radio, where advertisers geared their sales campaign to take off in September and relent by about May, when Americans presumably left their homes for the outdoors. They couldn't take radios with them then, and they certainly couldn't lug around tank-sized TV sets.

"This was before air conditioning," notes NBC programming whiz Paul Klein, an expert on TV history and economics. "People didn't stay indoors in the summer. The advertiser presumed that the winter season was the best time to sell his product. There were often reasons. The TV season coincided with the Broadway season; plays never opened in the summertime. And with department store selling patterns. Everybody started selling a week or two later Labor Day."

Television caught on quickly. When "Mama" made her 1949 debut, there were only about 1 million TV homes in the country. By September of 1950, the number had grown to 7.5 million. Today there are an estimated 71.2 million TV households.

"Eventually it started to appear to advertisers that they could buy cheap time in the summer when the rates were lower," says Klein. "For years summer advertisers like Coca-Cola have been getting away with murder."

Summer became a prime time for moneymaking, and risk-taking grew riskier with stakes higher. "Why, the most expensive program in the entire world is in the summer," Klein notes. He means the Olympics every four years. Otherwise, it's a summer snooze.

What happened to summer is like what happened to Sunday afternoons. In the '50s, on Sunday afternoons, even eggheads could watch TV without shame. It was the networks' good deed day. There were shows like "Omnibus" (with Alistair Cooke), "Wisdom," "The Seven Lively Arts" and "The Twentieth Century." Dave Garroway sauntered electronically and rather amazingly around "Wide Wide World." Ray Bolger danced and sang engagingly in "Washington Square."

Even the cartoons were faily intelligent: "Rocky and His Friends," with Bullwinkle J. Moose, from the innovative Jay Ward factory, and, from 1956 to 1958 on CBS, "The Gerald McBoing-Boing Show," a collection of eclectic animated shorts originally produced for theatrical use by the now-defunct but trial-blazing U. P. A. studios.

All that is gone. Sunday afternoon, once the numbingly predictable news interview shows get out of the way, is all sports-sports-sports. It's big money time! When the networks found that they could make a killing with ont football game on a Sunday afternoon, they got the football leagues to schedule games so there was time to telecast two in one day. That meant a double-killing.

Summer prime-time viewing never got quite so esoteric as Leonard Bernstein explaining everything, as he often did, on Sunday afternoons, but it was still an area more hospitable to minority interest programming than the regular season was. You can virtually forget about that condition ever recurring.

This year, if anything, the networks' summer programming is more half-hearted than ever. Only CBS is bothering even with the time-tested tradition of fashioning a frail summer frolic around a pop-singing personality. The criterion for this is usually one hit record.

So for summer '77, CBS offers six of one and half a dozen of the other: six editions of "The Marily McCoo and Billy Davis Jr. Show" (they sang "You Don't Have To Be a Star") and half a dozen of "The Starland Vocal Band Show" (Afternoon Delight"). At least the Starland show will include some on-locating taping. In fact, the Washington-born group will be taping at such Washington locations as Clyde's bar in Georgetown this week.

CBS has also given six chances for those insufferable mimes from (where else?) San Francisco, Robert Shields and Lorene Yarnell, to steal our hearts silently away with the "Shields and Yarnell show." The premiere last week broke the scoop that even the Captain and Tenille can be out-cutesie-woots-ie'd.

Also from CBS: six chapters of "A Year at the Top," the Norman Lear sitcom about old duffers who sell their souls to become young rockers (the show was announced as a mid-season replacement last year and then withdrawn); a musical-variety show featuring the sub-adolescent and sub-talented Keane Brothers; the TV premiere of Robert Altman's film "McCabe and Mrs. Miller" (June 22); and a real inspiration, old reruns of a flop NBC series called "The Family Holvak."

In the summer's early ratings, "Holvak" flopped again.

ABC is shifting some program time slots around as it reruns old episodes and has revived the former failure, "Westside Medical." NBC is airing a few more editions of a delightful and formula-bucking musical revue, "3 Girls 3," which premiered for a short run during the regular season, and will introduce one new series, a sitcom called "The Kallikaks," which the network calls a saga of "beating the system" revolving around "a Southern family operating a decrepit two-pump gas station in the middle of Nowhere, Calif."

The brightest hope, really, coems not from the networks but from independent producer Norman Lear. His new mock-talk show, "Fernwood Tonight," which replaces "Mary Hartman, Mary Hartman," stars Martin Mull as host of what is being called 'the first totally fictional talk-variety show on television." The program will premiere in Washington on Channel 20, Monday, July 4 at 8 p.m.

Advance reports call its satire at least initially hilarious. Of course, Norman Lear has failed us before. But he won't have to work very hard to beat the competition in the long, hot weeks ahead. Because summer television isn't what it used to be. It was never very good, but it has managed to get worse."