WITH A PLAY on words, a bit of Japanese technology and and a large dose of American gumption, the Postal Service last Friday began its long-awaited experiment of selling plastic stamps through automatic teller machines.

As Postmaster General Anthony M. Frank said in announcing the release of the country's first ATM stamp: "For years, customers have enjoyed the convenience of banking by mail. Now they will have the convenience of mailing by bank."

After a bank executive inserted a plastic bank card in a Japanese-made ATM at a Seattle bank last Friday afternoon, Frank received the first $3 sheet of the brillantly colored red, white and blue stamps. Wiping his brow in mock worry, the postmaster general quipped: "Let the record show, I never doubted it {would work} for a moment."

The stamps will be sold to the public at 10 locations in the Seattle area during the next six months in a test of the Postal Service's latest effort to find new -- and cheaper -- ways to sell its stamps.

If successful, postal officials hope to tap ATM networks across the nation and let bankers handle a growing share of the selling that long has been largely the work of postal clerks.

For the moment, however, the marketing venture presents a dilemma for both stamp collectors and postal clerks. For collectors, the obvious question is how will they collect the plastic stamps, individually or in the dollar-sized sheets of 12?

The cash-hungry Postal Service, of course, would prefer that they collect the full sheets and, since the stamps won't be sold individually, that's probably the way most collectors will save them. Given the limited number of stamps being printed for the experiment, 2 million compared to the typical commemorative run of 160 million, it seems certain that many collectors will hoard these stamps.

The problem for the Postal Service is one it learned the hard way last year, when it released another self-sticking stamp with an unusual appearance: Will postal clerks recognize the stamp as genuine and deliver letters bearing them?

Unlikely as it seems, the Postal Service's release of self-sticking stamps and a number of special souvenir sheets of stamps last year infuriated collectors across the country as numerous Post Offices and letter carriers refused to honor them as postage. Postal employees have been alerted to the plastic stamps, but odds are that the shiny stamps will stun a number of letter carriers and provoke controversy when they dun recepients for "postage due."

Plans for the new stamps were disclosed in a flashy video display titled "Post Office of the Future" at World Stamp Expo '89 in Washington last November. An ATM was wheeled into the show and, with a soft whirr, it dutifully spit forth demonstration sheets for Frank and senior postal executives.

Since then, the concept has undergone extensive testing on an ATM sold by Fujitsu Systems of America, a machine said to be the most demanding of any ATM machine in America.

The ATM stamps have to be as thin as dollar bills so the sheets can fit into the machine and be dispensed in the precise number ordered, as if they were money. If the concept can work in a Fujitsu machine, postal officials say, it can work in any other ATM.

The colorful stamps show a stylistic, diagonal view of a portion of an American flag and carry the lettering "25 USA." They were designed by Harry Zelenko of New York and were manufactured by Avery International Corp. of Pasadena, Calif., a leading plastic-adhesives company.

The stamps are less than .005 of an inch thick and yet have 10 layers of ink, polyester and various coatings. Postal officials caught a lot of flack from environmentalists after their initial announcement and since have downplayed the early description of the stamps as "plastic."

In announcing them last week, officials said that if the tests are successful, they will work toward development of a plastic material that can be recycled. They noted that the stamps can be soaked off envelopes in water for collectors who want used copies of the stamps.

But Frank, a former savings and loan executive, also declared that officials are optimistic that "a paper ATM stamp . . . can be developed." Results on the sales of a 25-cent paper self-adhesive stamp released last year are due to be released soon, according to a Postal Service spokesman.

Bill McAllister is a member of The Washington Post national staff.