YOU, A petless person in search of groceries, turn into a long aisle in the supermarket. Pet Food. You walk and you walk. You pass a dozen brands of canned dog food, the hanging leashes and flea collars. But it's not finished. Next comes the bird feed, then more for the four-legged pets: bags of dried dog food below, packages of pop-art hamburger likenesses above. Names such as "Good Mews," "Bright Eyes," "Fish Ahoy," demand your attention. Like a seductive restaurant menu, the shelf reveals a "Seaside Supper" and a "Gourmet Meal" (featuring "Liver, chicken and tuna flavors").

In this store the display takes up an entire aisle, minus a few feet where charcoal was packed in as the outdoor barbecue season began. And this is a supermarket in the District. They sell even more in the suburbs.

Pet people may not be surprised at the variety and amount of food, but the rest of us would. According to industry statistics, pet food was a $3.3 billion business last year, accounting for 7.13 percent of all food sales. That's nearly four times as much money as was spent on baby food, more than twice the amount spent on cereals, flour and macaroni products for people.

Of course Americans keep a large number of pets. A food industry estimate is 50 million dogs and 38 million cats, a sizable jump over the pet population of a decade ago. (The census isn't exact. The Humane Society of the United States recent statistic lists 23 million cats and 41 million dogs. For the first time an estimated 55 percent of all households in the United States own either a dog or cat or both. This breaks down to 33 percent dogs, 12 percent cats and 10 percent both.)

Another statistic of some interest is the $155 million that was spent on television advertising of pet foods in 1978. Those ads, and the ones in magazines and newspapers, aren't geared to pets, but to people. Pets can't read, which is too bad. If they could, they surely would reject a great deal of what is offered them on supermarket shelves without even tasting it. Because there is every indication that their owners are as susceptible to vague nutrition claims to commercial hype and packaging gimmicks, to cosmetic additives in feeding their pets as in feeding themselves.

Take, for example, Friskies Chunky Beef. It has 23 ingredients and only one is beef. Why is it "chunky"? Because that's the way dad likes his beef stew. Or the 29 in Alpo Beef Stew for dogs, including the potential carcinogen sodium nitrite, added, the label says "to promote color retention." Dogs, as we all know, are color blind. So only the owner sees the bright-looking color, while Fido gets to ingest the sodium nitrite.

Furthermore, in a time of recession, protein-prone pets are better than ever. Around the nation "premium" dog food outsells "maintenance" quality dog food in the canned category 68.5 percent to 32.5 percent, while cats, prime objects of pampering that they are, are fed "gourmet" cans over "maintenance" at a ratio of 86 to 14 percent. Generic brands are a new factor in what observers say is one of the most volatile facets of the food industry. "There's been a big influx of generics," said one salesman, "but so far they are cuttng into dry food sales, not canned."

This is probably the place to subdivide the genre. There are dry, soft-dry, semi-moist and canned pet foods, with separate formulas in each category for dogs and cats. Dry foods are the biggest sellers. In the past 12 months dry dog food has accounted for nearly $1.3 billion in sales versus sales of $745 million for canned dog food. Canned has been gaining, however, while semi-moist foods (prepared serving packets) fell off once word got round that the formulas contained a good deal of sucrose and made pets fat.

There's more money in dog food, but dogs eat more than cats. Cats, on the other hand, have a reputation as more finicky eaters. Morris the Cat would have intimidated Escoffier.

There are some big names in pet food, General Foods, Quaker Oats, Liggett & Meyers and Ralston-Purina among them. It's not all advertising, either. You'll find product development departments and test kennels. You'll find, as well, ingredient labeling and nutrition information on most products.

Dr. Terri McGinnis, a California veterinarian and the author of the authoritative "Dog & Cat Good Food Book" (Taylor & Ng), advocates a basic diet of commercial products that are certified "complete and balanced." aIn her view, "the average person has neither the time nor the money to cook for a pet."

She advocates buying pet food at the supermarket, but only after taking the time to read ingredient and nutrition information on the labels. For pets that do not have special diet needs, her recommendation is to use a combination of dry foods as basic fare and supplement this with some canned canned foods (which provides needed fat ).

On a day-to-day basis, she points out in her book, liquids are more important to a pet than solid food. Like most of their masters, pets tend to be overfed and underexercised.

Diet and recipe books for pets are a fad, she feels, brought on in part because of the concern about food additives, in part because humans are more aware of nutrition needs and perhaps because the percentage of expensive, pedigreed pets has increased.

"Breeding doesn't count for much," she said. "Dogs are dogs. But they need a balanced diet and few people can construct or cook one for themselves, much less their pets. Table scraps are fine, but only as a supplement. Pets don't need carbohydrates or vitamin C as people do. They go for protein but too much meat can cause problems.

"It's only in the past few years that there has been much information on animal nutrition available, even to vets. What I would like to see is a serious effort at education instead of all this advertising competition and gimmicks like making cat food in the shape of a fish or making claims for the amount of protein in a product without disclosing how good or bad the quality of the protein is."

That's not likely, according to industry sources. The competition is keener than ever. One brand, Kitty, is in bankruptcy; another, Jim Dandy, is reported to be up for sale, and Rival was sold recently.

"The business keeps changing," said one salesman. "There are constant promotions and new items (one estimate counts 1,000 different labels). Pet food makes a good profit for a supermarket, maybe 23 to 28 percent (against a store-wide gross profit average of around 20 percent), but candy does better. It's on sale most of the time."

Like other foods, pet products cost more in Washington. The salesman estimated that a large-size can of food selling for 59 to 63 cents in Philadelphia would sell for 67 to 69 cents here.

While you are searching those long aisles to pay those high prices, ponder what M.F.K. Fisher (who, in addition to her gift for graceful prose, can see beyond her stove) wrote in "How To Cook a Wolf": "Myself, I have always said (and practiced) that I would never give a dog or cat what I would not eat myself."