TOKYO, FEB. 12 -- The Japanese, having turned transistors, television sets and small automobiles into huge commercial successes, are now busily at work on another western idea -- Valentine's Day, which in the last few years has taken this country by storm.

Like many other successful imports from the West, including Christmas and more recently Halloween, it is being adapted in decidedly Japanese ways.

Of course, red hearts remain the motif, and plenty of cupids dangle on strings from the ceilings of stores. Valentine's cards do exist, often written in English, because it is considered chic.

But in Japan, this day is as much about duty and obligation, those crucial underpinnings of Japanese society, as it is about romantic expressions of love. And, unlike in the United States, only women do the gift giving, delivering chocolate, primarily, to the men around them.

Thus, all week long department stores, boutiques and sweet shops have been filled with female office workers buying tiny boxes of giri choco, or "obligation chocolates," for male bosses and colleagues.

Wives have been buying chocolate golf clubs, cigarette packs and plates of chocolate ham and chocolate eggs for fathers, husbands and sons. Young women in love have tended toward more romantic gifts, such as finely crafted stems of chocolate roses, or more ribald ones that cannot be described here.

Chocolates, rather than roses, are the chosen medium for expressing Valentine's Day sentiments because the chocolate industry brought the first valentine to Japan.

The Mary Chocolate Co., which claims it was the one to spot the commercial potential of the day, sold its first chocolate hearts in 1958. According to a spokesman, the company was eager to push Valentine's Day because February traditionally has been one of the slowest sales months of the year.

Initially, many Japanese were embarrassed at the idea of such a public display of sentiment, and Valentine's sales for Mary Chocolate amounted to only 500 yen the first year.

This year, Valentine's Day posters went up the day the Christmas ones came down and sales for the chocolate industry are expected to exceed 40 billion yen, or more than $300 million. That accounts for about 10 percent of annual chocolate sales and means that on average every single Japanese male between the ages of 11 and 59 will receive 1 1/2 valentine gifts.

Many more exotic, and pricey, goods are for sale than anyone dreamed in 1958. At the Matsuzakaya department store in downtown Tokyo, for instance, a special Valentine's Day corner boasts chocolate pianos, huge red-foil-covered chocolate lips, and hearts of every color bearing such inscriptions as "I love you, daddy" and "Thank you for everything you do for me."

In addition, the store has on display a three-foot high, 110-pound solid chocolate reproduction of St. Valentine's Cathedral in Terni, Italy. Worth more than $2,000, the chocolate cathedral will be given away in a raffle.

In the last few years, gifts other than chocolate have begun to appear, such as $8,000 solid-gold hearts and clothing decorated with hearts. But, if a recent visit to a special Valentine's Day shop is a good indicator, the most popular items remain small, inexpensive packages of chocolate.

Yoko Kumaki stood in line recently to pay for a shopping bag overflowing with small boxes of chocolate that she said were for her father, husband, father-in-law, sons and men she works for and with at the neighborhood supermarket.

All told, she spent less than $50, including a few expensive items for her family. The rest were small boxes of giri choco that all the men at work have come to expect will appear on their desks on or about Feb. 14 as a token of thanks.

"I don't spend a lot on them," she said, "But they expect it so you have to give something or they will get upset."

Exactly why only women give Valentine's Day gifts in Japan is a little murky. But other candy makers, envious of the chocolate industry's February profits, are eager to correct the situation. A few years ago they began promoting March 15 as "White Day," when men should give gifts of candy to wives, girlfriends and mothers. Whether it will catch on is still an open question.