AS IT DOES in the States, the tinkle of the ice cream wagon's bell brings children flocking from the crowded houses along the canals, from the street markets and tiny alleys of this jammed city. But this ice cream wagon asks a differnt price for its wares. Any child may have a free Popsicle if accompanied by a parent willing to accept a condom, a month's supply of birth control pills or a vasectomy.

The gaily painted wagon which carries both ice cream and a mobily birthcontrol clinic is the latest of a vast array of lures thought up by MechaiMechai Viravaidya, an economist turned supersalesman. The 38-year-old Mechai, graduate of the University of Melbourne, cousin of the king of Thialand by marriage, son of a Scotch mother and a Thai father (both distinguished physicians in Bangkok), has taken on, with evangelical fervor -- and much humor -- the task of reducing his country's birth rate. And he is succeeding. hIndeed, the local colloquialsim for a condom is a "Mechai."

In a country burdened with too many people for its meager resources and recurrently inundated by Indochinese refugees, the limiting of new births is crucial.

In the early '70s, the birth rate of Thialand, like that of most third world countries, had skyrocketed. The combination of more food from the new grains of the Green Revolution and improved health care resulted in far fewer infant deaths. The population was about to double in a 20-year period. Rural families traditionally had 18 or 20 children, but before World War II many children died of tropical diseases and infections. Now most live. "We became," says Mechai, "a nation unable to provide for our children. Only half the children in Thialand could attend school even though the schools went on double schedule.

This year, the statistics show what nationwide planning can accomplish. There were 41 percent fewer pregnancies than five years ago.A recent questionnair sent to 8,000 villages found that there were almost no unplanned births.

"When I was in the government planning office," explains Mechai, "with my degree hanging smartly on the wall, I was director of a team that made marvelous plans for Thailand.

"I was that we were advancing economically, but all our gains were being eaten up by overpopulation. I realized we would have to adopt a radical new approah to this most terrible problem. We had to change the Thai's puritannical view of sex, make the whole business of how many children a family had a subject easily discussed and thus acted upon. I wanted to remove the taboos, take birth control out of the realm of the secretive, make it fun."

In 1974, Mechai founded the community-based, nonprofit Family Planning Services and launched his extraordinary campaign to curb family size by handing out his calling card everywhere, from formal state dinners to vendors at the city market. It is a brightly colored contraceptive attached to an order form which featuress a Buddhist scripture, "Many births cause suffering." An alternate card tells people just how much an extra child will cost them in a series of drawings explains use. The tear-off card bears this commerical: "Plan your family with rainbow colored Mechais. Birght colors, safe, easy to use. Reduce anxiety. No side effects. Highly effective family planning . . ."

In less than a decade, what was once the province of staid physicians, a subject eschewed by peasant families and certainly not mentioned in polite circles, is part of every day Thai life as a result of Mechai's strenuous and imaginative campaign.

Contraceptives can be bouth from rice farmerss, taxi drivers and hotel porters. Bus fares, theater ticket and tips can be payed in them. The national soccer team wears a shirt emblazoned with the motto, "Too many children make you poor." In rural areas, new shipments of contraceptives are blessed by Buddhist monks before they are distributed.

The interest on a bank loan may be halved by the borrower if his wife is sterilized or he has a vasectomy. In other villages, baby pigs are given to those who practice family planning. "Accomplishing two things at once," says the energetic Mechai, "improving the diet and popularizing birth control."

School children throughout Thailand sing a song, based on a popular tune, that describes the hardships of having too many children and thus not having enough for everyone to eat. It ends with a happy refrain reminding everyone there is no need to worry as the local family planning agent can tell their parents where to get a condom or an I.U.D. or a vasectomy.

"If parents hear this often enough from their children it becomes part of their thinking," says Mechai. "Anyway, it is good to give these kids an early start. They will to be parents soon enough."

In a field, Mechai operated like an old-time medicine man. He pulls up his van near a country market, spreads out his wares: T-shirts, bikini pants, bed sheets and pillow cases all bearing the message that too many children make people poor.

There are also sideshows where ovulation charts are used as dartboards, and other games demonstrate devices.

The audience is ragged --- but eager.

Later, Mechai explains his merchandising technique to a foreign visitor. "Once people accept the idea that contraceptives are just one more item you can buy easily like soap or toothpastes or dried fish, they will be more likely to use them. Thais are a thrifty people, if you can show many uses for a product you are more likely to get a buyer. If I can accomplish that by blowing up balloons or filling condoms with rice I will do it. Acceptance is the name of the game.

"There was a time," Mechai adds with evident pleasure, "when my methods shocked people. I verged on becoming a social pariah.It was especially awkward for my wife, who is the personal press aide of the King of Thailand." a

The apex of Mechai's shock program came in 1975 when he crashed the first state dinner given for the North Vietnamese vice-foreign minister by the Thai government. Thai officials, already tense about the occasion, were traumatized when they entered the formal dining room to find the ubiquitous calling card placed carefully at each place. Mechai appeared personally to pass out T-shirts with birth control messages printed across the front. The austere Vietnamese, thinking this was Thai custom, dutiffly donned the shirts and subsequently asked Mechai to discuss his program with them. "I told them," he says with pride, "that population control is the most important war to win. Their current answer, exporting human beings wholesale, is not what I had in mind."

Now Mechai has become something of a national hero. The mention of his name hastens visitors through customs, brings a waiter scurrying in Bangkok restaurant, guarantees space in crowded provincial hotels.The Thai government has adopted many of his ideas and vastly expanded its own family planning programs. "Mechai," says in American observer, "leads the way. He persuades someone to have his vasectomy televised, everyone is officially shocked and a few weeks later the number of vasectomies being performed in government clinics has quadrupled."

A diplomat's wife regales the vistor at a large dinner with a description of how Mechai celebrated International Women's year by inviting middle class ladies of Bangkok to donate $10 to "vasectomize the man of your choice." It worked, said the foreigner. "And what's more, some of us joined in the game." w

One wonders what makes Mechai run, but run he does, all the time. During one lunch he gave a formal scholarly talk on the concept of community health and nutrition centers as a starting point for rural social change. A moment later he described with glee how he had found a way of perfecting the size of the product that carries his name -- by enlisting the aid of Bangkok massage parlors.

"I am, in the vernacular," he says, "a good-doer, but I abhor the cant. I think the words of economists and sociologists often obscure the objective. For example, what does "World Population Year' mean? What we should have is a simple, "No Baby Year." You can do anything in a country if you let people have fun learning about it and let them do things in their own way. If you stay away from preaching, from theoretics, and allow people to do one thing at a time, you can bring about, slowly, real social change. We don't need to sent social workers to our villages. The villagers can be their on social workers once they understand the pragmatic advantages of progress, be it fewer children, better nutrition or better health."

There are days traveling with Mechai when you think you can't abide one more imaginative gimmick, like having a school band play the national anthem for the first child who swallows all the dreadful tasting anti-parasite medicine. But then as you progress through the villages, you see that in the wake of the medicine man, with his wares, his squealing pigs and worshipping young staffers, there is solid method to his apparent madness.

"I looked," says the social reformer, "for ways to get participating that involved little training and less money. I decided sex was a good place to begin. You don't need a Ph.D. to understand it."

Mechai's organizationn supplies contraceptives but insists local people be the providers and motivators. "And once their credibility is establihsed we are able to move into a series of health and development programs. We regard the family planning program as only the first step in a long war. It teaches people that if they participate it works to their benefit."

"The consumption pattern in poor countries like Thailand is such that people don't see immediate benefit from not haveing a fifth or sixth child. We set out to provide some immediately tangible benefit from taking up family planning -- almost as fast as they swallow that pill. We call it our 'better market' program."

Participants in the program can use the family planning organization as agent to sell their nonperishable products like coconuts, pumpkins, silk and handicrafts.

"We cut out the middleman," said Mechai. "We find the markets, arrange the transportation, and the villagers get at least 30 percent more for their produce. How's that for proving that family planning is worthwhile?"

Mechai is proud that his small clinic in Bangkok and the expenses of his field staff are more than half paid for by the sale of products. The rest of his money comes from grants from the Population Crisis Committee, International Planned Parenthood, and a Japanese government-sponsored foundation. The Agency for International Development has given money for specific projects.

"If other third world countries had a Mechai," says Frederick Pinkham of the Population Crisis Committee in Washington, whose organization finances family planning throughout the world, "we would see a substantial drop in population figures.It takes someone as tireless and imaginative -- with the kind of access to power -- that Mechai has. One can not avoid the fact that a man already established in his own society can take more radical action and remain acceptable and thus effective. There are very few social scientists anywhere who would be plastered on every taxi-cab advertising a contraceptive. It takes a special self-confidence and rare dedication."

Mechai will be in this country next week to speak at a Planned Parenthood Federation celebration of Margaret Sanger Year in Houston. He will be in Washington to discuss financing of his latest scheme -- a solar oven capable of purifying water for an entire village and producing enough alcohol to fuel village machinery.