It is Christmas at America's biggest toy merchant, Toys "R" Us.

Strolling down Aisle 4 in the Tysons Corner store, the shopper finds an assortment of toy musical instruments, among them five different marching drums, 30 varieties of guitars, 28 makes of harmonicas, kazoos, melodeons, trumpets, saxophones, accordions and flutes, 12 kinds of pianos and eight xylophones. There are 41 model-car and motorcycle-racing sets, 22 dart games, six weight lifting sets, four kinds of boxing gloves, seven varieties of punching bags and a rack of 17 types of roller skates.

"What we are is a supermarket for toys," says Toys "R" Us president Charles Lazarus. "We don't have a competitor in variety. There is none."

Toys "R" Us is a toy store with a message: the philosophy is discount merchandising uniform down to the candy-striped smocks of the high school students who make up the seasonal help, and the Electronic Paramedics are stacked in section 313 at Tysons Corner just as they are in section 313 in Milwaukee.

The stores are warehouses, without elaborate Christmas displays and with only one thing in mind: the biggest selection and lowest prices anywhere. Like McDonald's, with its regimented service and standardized burgers and fries, Toys "R" Us has become an American insitution.

"We love the comparison," says Lazarus, who started the toy empire by selling children's furniture at 18th Street and Columbia Road NW more than 30 years ago. "Except we think they're a little like us. We were doing it first."

And like the Big Mac, it has sold.

An estimated four out of 10 toy shoppers buy at the 120 Toys "R" Us stores around the country, making the chain a $600-million-a-year business and sending its stock from 25 cents a share in 1975 to almost $31 today. Toys "R" Us, including the Tysons Corner store, which is designed exactly like every store in the chain, is the prototype of the mass mercantilism that has become the American Christmas.

Owners of smaller, more traditional toy stores grumble that Toys "R" Us sells items they wouldn't put on their shelves. "All this is part of what I see as the bad part of the American Way," says Robert Joy, owner of the Red Balloon in Georgetown. "Create a demand for an item, create the item and then sell as much of it as you can."

Lazarus says Toys "R" Us actually has expanded toy selection and opened the market to the less affluent. About 30 years ago, the domestic toy market was small and many toys were imported and expensive, he says. "Most toys were for rich persons . . . . What we are offering is the best quality at a moderate price."

The massive quantities Toys "R" Us buys from manufacturers allows the company to pass on big discounts to customers. Though not always the lowest around, most of the chain's prices are 20 to 50 percent below list. Last year, the company made $29 million in profits on sales of about 150 million toys.

In the Saturday crush at the Tysons Corner store, housewives in fur-lined parkas swarm around the "check approval" desk. Grandfathers look thoughtfully at a row of Tonka trucks. A toddler wrapped in a blue snowsuit with a pointed hood grabs a music box off the shelf, only to give it back to Mommy after a tug-of-war. One customer is a lawyer dressed in blue blazer and tassle loafers. Another is a truck driver in bib overalls.

Merchandise stacked high on a hand truck rumbles through the aisles, and stockboys contest with customers to keep the shelves filled. But even the company that tries to keep enough for everybody on hand has been in a mild state of shock over the popularity of some items such as Strawberry Shortcake, a 5 1/2-inch doll in a red and white dress and pink bonnet that last year sold to the tune of $100 million, or about 10 million dolls. At Toys "R" Us, the toy sells for about $5, about half the national list price.

"Where's Strawberry Shortcake?" one middle-aged woman asks a clerk.

"I guess we don't have any right now," he says.

"Do you know when you might?"

"Well, we get trucks in almost every day. It could be tomorrow. It could be three days before Christmas."

Most of the shoppers are adults and on weeknights, when the store is open until midnight, couples who have left the kids with baby sitters peruse the mountains of toys that clerks -- who are known as "mad mounders" -- stack 14 feet high. Some shoppers carry lists in little spiral notebooks, just like the giraffe family in the company's televison and newspaper ads.

In fact, kids can't even buy toys at Toys "R" Us. "The Children's Bargain Town" is closed to children under 16, unless they first come in with a parent and sign out an identity card that allows the child to shop alone. The company doesn't want complaints from parents who've had money stolen by toy-hungry offspring, says Washington regional manager William Bederman.

The Tysons store sells about $6 million in toys each year, about half of it during the Christmas season. Around Thanksgiving, the gym sets, swimming pools and other warm-weather items are shipped out to make room for truckloads of Dizzy Dolphins, Speak 'n Spells, Space Invaders and thousands of other toys.

This year, hand-held computer games, so popular in the late 70s that computer chip manufacturers could hardly keep up with demand, have given way to more expensive, electronic television games such as Atari and Magnavox Odyssey. Last year, roller skates were hot. This year, the complicated imagination game of Dungeon and Dragons is selling big.

Each toy is tracked from the time it leaves the manufacturer to the time it's bagged for the ride home. Every Incredible Hulk sale is registered at the regional computer in Beltsville 30 miles away and relayed by satellite to company headquarters in Rochelle Park, N.J., where 15 buyers make sure there are enough Hulks to go around.

Pilfering of hand-held computer games prompted the company to put them all behind a glass enclosure near the cash registers and led to a typically bureaucratic purchase procedure. In Aisle 1, sample computer games now hang from the ceiling with tags shoppers can tear off and take to the front desk to make a purchase. Among the selections are seven computer baseball games alone, to say nothing of basketball, soccer, football, horse racing and others.

"It's mind boggling," says Fairfax lawyer Clark Kattenburg, who was shopping for his two sons last week. "I have to admit, my first experience here was that my brain would short circuit. It was overwhelming."

Bowling enthusiasts can throw strikes and spares on a computer game that attaches to the wrist like a watch. Another toy attaches to wrist, forearm, bicep or calf and uses muscle movements as a "mysterious force" to control a lunar landrover called Radarc.

An assortment of toys seem to imply that riding a car or motorcycle is more fun when the driver is flying through the air or possibly crashing into another car or motorcycle. They have names like Criss Cross Crash, Trailer Cutoff and Maniac Madness. An electric toy called Sound Gizmo makes noises like an airplane taking off or wailing police sirens -- so you can "tell 'em the cops are coming."

"Three or four years ago, the top dolls were Cher and Evel Knievel," says Joy of the Red Balloon. "A number of sane adults would say . . . we've got to do something. Because if we're setting kids up with Cher and Knievel as heros, then something is really wrong."

Says Lazarus: "We give our customers what they want."

"Look at all this junky girls stuff," says a 7-year-old boy to his older sister as he holds up an evening gown for a Cher doll.

"It is not," his sister says. "Why don't you go find something for yourself and leave us alone."

Many of the customers have come to know the Tysons Toys "R" Us by heart, which is exactly what the chain's marketing experts hoped for when they insisted that every aisle of every store in America be stocked in exactly the same fashion. "People who move from San Francisco to Washington can walk into a store and know exactly where to find something," says Lazarus. "It's fantastic! They love it!"

"When the boys were growing up, we didn't have to worry about any of these other toys," said Patricia Kattenburg, explaining her family's transition from shopping in the "toddler corner" when her children were young to scanning other rows as they grew older.

"After a while," she says, "you get to know your aisle."

Still, some shoppers find Toys "R" Us exasperating. One mother, dressed in a black afghan sweater and green corduroy pants tucked into her cowboy boots, gazed disapprovingly at a display of Empire Strikes Back toys.

"Where's the robot?" she said to no one in particular. "And the hairy monster? It just seems like I can get Kelly something better than this plastic."

Other toy store owners make similar criticisms of the quality of Toys "R" Us toys, saying that because the chain depends heavily on the multimillion-dollar advertising of major toy manufacturers, it is tied to the toy industry's appetite for toys with short life spans. The stores' line of educational and safety-minded toys is extensive, but it carries little in the way of hand-made or imported toys that stores like the Red Balloon or Granny's Place in Alexandria consider their specialty.

At Granny's, says owner Roger Wall, you can still buy a hand-finished, wooden rattle from Germany for $1.95. "I would put the quality of this item against whatever they have at the price," Wall says.

But for shoppers such as Sylvia Perucci of Vienna, who's been buying her toys at Toys "R" Us for 10 years, it's the easy parking, check approval and generous refund policy that make Christmas at Toys "R" Us worthwhile.

"Once, I bought an electric football game here and it fell apart after an hour," she explained. "I came back and got a different brand and that one fell apart, too. Finally, I came back and got a third one and that worked just fine."

And if the success of Toys "R" Us has been at the expense of old-fashioned toy stores and the elaborate holiday displays and decorations that signaled Christmas to generations of shoppers, at least Perucci isn't complaining.

"There isn't any room for decorations here," she said, as she searched unsuccessfully through 32 feet of dump trucks, tractor trailers and fire engines in Aisle 2 for a particular rescue truck she had seen on the shelves years before. "It's all packed with toys."