Gambling again that smaller is better, Japanese electronics companies are putting the first 8mm home video systems on the market, despite uncertainties about the new format's reception with consumers.

The new systems are not -- for the present, at least -- intended as replacement for the bulkier VHS and Beta equipment that is now in some 7 million U.S. homes.

But some industry visionaries, looking a decade or so down the road, wonder if the smaller, lighter and more convenient 8mm video will overtake today's two video systems, which use tape that is a half-inch wide.

For the start, sales will be limited. Marketing campaigns will pitch the new 8mm video tape equipment as replacements for the 8mm movie cameras and projectors that a generation of Americans used for home movies.

The business is fast shaping up as one more that Japan will dominate worldwide. Japanese companies now have the only production facilities (there is talk of some in South Korea) and have begun supplying U.S. companies.

Here is what the Japanese majors are doing:

* The country's largest electronics manufacturer, Matsushita, known in the United States by the Panasonic, National and Technics brands, is making 8mm units for Eastman Kodak Co. and General Electric Co. Kodak put the first units on sale in the United States last September.

* After ending discussions with RCA Corp. over a production deal, Hitachi is watching the market before committing itself to marketing under its own name. Sony this month became the first company to market an 8mm product in Japan. Company officials say it plans to make 30,000 units a month, 20,000 for its own label and 10,000 to go to Fuji Film.

* Sanyo is introducing 8mm video equipment in the United States under its own name. It also is discussing production arrangements with other companies.

* Toshiba has shipped equipment to Polaroid Corp. for sale in the United States. But it, too, is watching conditions before deciding whether to enter the market for itself.

Sony finished work on a prototype 8mm system in 1980 and offered it to the industry as a standard. In April last year, following lengthy negotiations, 127 companies signed an agreement creating a single standard based on Sony's.

That cleared the way for production. But in some analysts' view, the biggest interest was not from the electronics giants, who already were busy turning out some 30 million half-inch VCRs a year.

Rather, it was from film and camera companies. "The 8mm film business is in decline all over the world," noted Tohru Arai, spokesman for the Electronic Industries Association of Japan. "To reverse that decline, they needed a new product. They seized on 8mm video."

The so-called private-label business will enable producers to make assured sales for themselves and to accelerate introduction and acceptance of the new product by tapping into others' distribution networks, it is hoped.

Japanese electronics companies continue to feel competitive heat from South Korea and other newly industrializing countries of Asia. Already, the newcomers are taking over lower-scale electronics production -- digital watches, color TVs, for instance -- that was once a virtual Japanese preserve.

For the long run, 8mm is viewed as a logical upward progression, which will apply Japanese companies' modern plants and skill at miniaturization to full advantage. Sony, for instance, managed to reduce circuit board size in its product by 40 percent compared with its smallest portable Beta recorder.

Capital expenditure is comparatively small for companies already heavily committed to half-inch equipment. Some components are interchangeable. Eight-millimeter production is generally proceeding alongside half-inch.

The single 8mm format represents a substantial advantage. Owners of half-inch systems have been bedeviled by the competition between two incompatible systems, VHS and Beta. Beta was the first to the market in 1975, but its share of sales has slipped to between 20 and 30 percent.

Another advantage is weight and size. Sony's product, selling in Japan for about $1,120, weighs less than 4 1/2 pounds and combines camera, recorder and player into a single unit. The owner can plug it directly into a TV set to watch the tape.

Eight-millimeter equipment is also able to record sound digitally, giving the potential for higher quality sound.

But there are major drawbacks, too. One is picture quality. It cannot match the clarity of half-inch systems, which consume about twice the tape area to generate an image. "Japanese consumers demand high-quality color and picture," said a Toshiba spokesman, explaining that company's wait-and-see approach to production.

Recording capacity of the 8mm cassettes -- they are slightly smaller than a conventional audio cassette -- also is a problem. Currently, the longest-playing one now available is good for only 90 minutes.

Sony launched its "Betamax" system in 1975 with cassettes of only 60 minutes' capacity. The longer playing time of VHS tapes that followed has been credited with much of that format's success. Sony, determined not to repeat that mistake, says it will introduce a two-hour 8mm cassette in several months.

But probably 8mm's biggest challenge is that it is coming into a world already full of VHS and Beta. The new cassettes are unusable on half-inch machines. (It is fairly easy, however, to transfer material from 8mm tape to half-inch tape.)

That is part of the rationale for starting sales with portable home movie equipment. Decks for off-the-air taping or viewing of pre-recorded cassettes will come later.

Reports of a "new video war" already have appeared in the Japanese press. By some analysts' accounts, Sony is anxious to promote 8mm to move away from reliance on the commercially disappointing Beta system.

But Sony denies any plans to dump Beta and will keep working on better picture and sound.

"At least for another 10 years, 8mm and Betamax will co-exist," said Kiyoshi Yamakawa, senior general manager of Sony's corporate video group.