Hardly had the buyers been shuttled into the darkened chamber for the presentation of yet another toy, than the demonstrator innocently announced that Star Rider would be "the hobby horse of the 21st century."

Lights! Fanfare! Film clips! Enemy ship approaching. Sights! Phasers! FIRE LASERS!

Even as the noise of the demonstration subsided, an overhead flood bathed the plastic spaceship in pure white light, while the Star Rider operator pointed out the salient indestructible features of the craft.

"This is really designed for the 3-year-old," he said, "but we've had up to 120 pounds on it with no problem."

Outside the chamber other salesmen were deftly explaining flying saucers with electric gyroscopes, tiny toy soldiers with built-in electronic synthesizers and a robot designed to interact with 3-and 4-year-olds, all part of the Playskool line of torys available for Christmas, 1979. While the company's biggest-selling items were once wooden blocks and carved toy trains for toddlers, this year it expects its Alfie the Robot to go to the head of the sales class.

And so it was at many of the 800 displays in the 76th Annual Toy Fair here last week: everywhere an encounter with some form of electronic intelligence. Spurred on by major technoligical advances, toy manufacturers have made some of the most creative uses of microelectronics of any industry. The public has responded voraciously: In the 18 months since the first hand-held units were introduced, sales have doubled every third month.

Last year Mattel Electronic Football became the biggest-selling item in toy history, grossing about $25 million at wholesale level in a single year. And the toy industry now predicts that electronics will bring in an additional billion dollars' worth of business this year, without detracting from other sales areas.

"The tremendous clean-out we all experienced last year was astounding," said George Ditomassi, Milton Bradley senior vice president for marketing. "We're seeing a new world opening for the toy industry. Far more 20-to 40- year-olds started coming into tou stores last year, especially males. And not only are older people buying toys; younger kids are using the same toys and they're displaying a sense of sophistication we never would have dreamed of a few years ago."

"The problem with all these electronics," said Stevhen Miller, a 30-year-old self-described toy nut, "is what they will do to kids' imaginations. Toys have gotten terribly literal, very closed in their functions, which is very antiimaginative. We've all talked about how we fear a robotized society, and I wonder if that's just what these toys are going to do to kids." 'Almost Frightening'

"it's almost frightening to think," said Jeff Rochlis, president of Mattel Electronics, "that for Christmas of 1977 there were only four electronic items: Battleship, Comp IV and our football and auto race games. This Christmas there will literally be hundreds."

Indeed, parents who struggled in vain last Christmas to locate the electronic Simon game that Milton Bradley was forced to allocate may well find their problems severely compounded this year, searching for:

Phaser Guns, modeled on the Star Trek movie, that emit infrared bursts coupled with synthesized audio. Hitting another gun results in a ricochet sound; a direct hit on the sensor triggers an explosion and shuts the other Phaser down for nine seconds.

Microvision, a Milton Bradley innovation using liquid crystal displays to replicate TV pong games in a hand-held $30 unit. Interchangeable chips allow the player to select any one of a half-dozen games from Space Wars to bowling to pang.

Wildfire, a hand-held pinball game that uses light-emitting diodes to recreate the action of bumpers, flippers and balls. Synthesized sound duplicates the noise of the real game, and the machine even "titls."

Brain Baffler, a hand-held game with a complete alpha-numeric keyboard that scrambles words, plays hangman, flashes anagrams, shuffles letters randomly and plays a total of six word-based games against either the computer or another player.

Big Trak, a computerized vehicle that can be pre-programmed with an internal keyboard to carry out any combination of seven differnet functions for up to a half mile of distance.

Electronic Football II, a more sophisticated version of the Mattel original, which sold over a million units last year. This one adds more players to the field, allows passing and lost yardage.

Zodiac, an astrology computer from Coleco that actually figures in a matter of 10 seconds the positions of noom and planets for any birthdate, time and place entered into the unit. A separate book is then used to figure horoscopes. Another unit, the Mattel Horoscope Computer, gives direct alpha-numeric predictions for specific dates entered in it. Baseball and Taxes

And then there is Mattel's IntelliVision, called "Staggering" by Robert Doyle, an inventor who works for rival Parker Bros., and "amazing" by Dave Eisenberg, a buyer from People's Drug in Alexandria.

IntelliVision is the first home computer that doesn't have to be programmed and provides as many game functions as it does record-keeping programs. While the system is displayed on the home color TV screen, the resolution is precise enough to let baseball players' caps show. Runners can lead and be picked off. Balls have to be fielded. Quarterbacks can pass and forwards can be fouled. Blackjack players can doubledown, backgammon players have a doubling cube.

The computer network is even more complex. One cassette will record family financial records, compute the most economical way to file taxes, and then display a 1040 form with the blanks filled in. Another cassette will plan a family shopping list with a diet or budget in mind. There are also childrens' learning games, speed-reading lessons, French lessons and even a programmable exercise program, with an animated instructor leading the viewer through his calisthenics.

The entire unit will sell for less than $500, with more memory capability -- 16,000 bytes -- than currently available for twice the price.

"In a few years you'll be ordering from Sears with this system," claimed Mattel's Rochlis, "have game programs delivered by telephone, even receive mail. We can transmit an entire newspaper to your home over phone lones in 1.5 seconds, and store a 1,200-page book on one regular cassette."

"Children are just going to have to learn to play in a different way," said Frances Noah, a buyer for two small toy shops in North Carolina. "I came here looking for some simple imported toys and I'm going home with orders for domestic electronics. I guess kids are going to be a lot faster and a lot smarter, but somehow I feel like they're missing out on a whole realm of play."

In 1977, Americans bought a respectable $21 million worth of electronic toys. Last year the figure jumped 500 percent to a $112 million share of the $3.8 billion toy industry. This year tay manufacturers expect electronics to register a staggering tenfold increase, and push the industry into the $5 billion sphere.

"My problem," said William Lance, a buyer from Charleston, W. Va., "is that even if you want to sell the kid a more imaginative toy, he wants a Simon or a Merlin or whatever else has been exposed on TV." 'Kids Expect Things'

"The reality of it," said Allan Rieper of Montpelier, Vt., "is that kids today are more accustomed to technological breakthroughs than we are. They expect things that we have a hard time imagining. I don't know how bad that is, except that when they get to be 15 they may expect a vacation on the moon"

The technology already has evolved considerably since last year, from the sublime to the frankly extraordinary. Kenner, for instance, has installed a simple heat sensor in the rear of a motorized dog called Bingo, which causes the puppy to run away when the child tries to catch him. Texas Instruments has developed a simple plug-in chip that expands the vocabulary of its synthesized speech Speak & Spell unit. Playskool has fitted an electronic gyroscope into a flying saucer that switches on as soon as it is lifted from the floor.

Schaper has installed a tiny nicad battery into a hovercraft that can be recharged in two minutes from D-cells in its launching berth. Kenner has developed a concealed muscle-sensing device that straps to the arm and operates a radio controlled vehicle. And Milton Bradley's Microvision as well as Mego's Minivid present the first use of liquid crystal display in hand-help units.

"I think we'll see two major fronts open up in the next two years," says inventor Doyle, the creator of Merlin and Wildfire, as well as two electronic board games, "color electronic display and voice recognition devices, as well as a real increased use of voice and sound synthesis. Why shouldn't kids of all ages love this stuff? We're all computers. We're all littled adding machines."

"Electronic chips have been the most incredible introduction to the toy industry since plastic." says Coleco's president, Arnold Greenberg. "By next year you'll see computerized dolls, trains, board games galore. This generation is a generation that's having a love affair with electronics.These kids will be much more scientific and more dexterous than any other generation. Our real problem now is that technology is way ahead of marketing.'