The schoolchildren here had been warned by their parents: If a strange car pulls into your schoolyard, it is the "family planning people."

The children would later say that their parents had warned them that these family-planning people would give away milk laced with birth-control drugs, that they would administer injections that would make the children sterile.

So when a strange car pulled into the dirt yard of Mugumoini Primary School here last month, the children took off. Hundreds of them fled their classrooms, some scrambling out the windows, according to a school official.

"They came running out of the school and sat on the street and cried," said a health official here. "They were saying that the boys would to be injected in the behind and the girls in the navel."

The children of Thika, a town in Kenya's highland coffee country 25 miles north of Nairobi, have calmed down and are back in class. But the rumors that sent them running have triggered what family planning officials here say is the most heated birth-control controversy in the history of Kenya -- an East African country whose 21 million people have recorded the highest population growth rate in the history of the world.

The rumors have infuriated President Daniel arap Moi, who has said that slowing down Kenya's population growth rate is the country's "most crucial challenge" and who personally introduced the school milk program in primary schools. Moi also is an enthusiastic backer of a five-year children's immunization program paid for by the World Health Organization and the Italian government.

Denouncing "rumormongers" who have slandered the immunization and school-milk programs, Moi has said that "unless they stop their dirty tricks, I will be forced to expose them publicly because they are enemies of the state." Moi told school officials to drink milk in front of children to show them it was safe.

In addition, according to the Kenya Times, the official newspaper of Moi's ruling party, the president "directed Kenyan police to arrest anybody on the spot found spreading rumors over the school milk scheme."

Last month the government arrested a stonecutter and a Roman Catholic priest for what the Kenya Times called "spreading malicious rumors over school milk." The government decided this week not to press charges against the priest, who was alleged to have told his congregation that some rich countries were helping Kenya ruin children by giving them birth-control drugs through milk. The priest was warned, however, that he could be arrested and charged again for the same offense.

The uproar over contraceptives and children comes at a time when the Kenyan government is beginning a four-year, $50-million program to step up awareness of family planning and increase the availability of contraceptives in Kenya.

It also comes at a time when the Kenyan government, which since the mid-1960s has been the most outspoken in Africa in its support of family planning, has begun to record a significant increase in the use of contraceptives by couples of child-bearing age. According to authoritative survey figures here, contraceptive use among couples has more than doubled in the past six years, from 6.7 percent to about 15 percent.

According to Gary Merritt, a demographer with the U.S. Agency for International Development (AID), the family planning program has begun to dent Kenya's record-breaking population growth rate. He said the growth rate has fallen from 4.1 percent in 1979 -- a rate that would double the country's population in less than 17 years -- to about 3.8 percent. The government's goal is to cut the growth rate to 3 percent in the next three years.

"This controversy plays on the suspicions even of educated people here in Kenya," a senior official of the International Planned Parenthood Federation here said. "It will scare people away. It will scare people who already participate in family planning to drop out."

According to AID, which bankrolls about one-third of the Kenyan family planning program, the flap about contraceptives in milk also coincides with sensitive negotiations between the Kenyan government and AID to market contraceptives donated by AID in stores across the country. In most of Kenya now, contraceptives are available only from hospitals and family planning clinics.

"The timing could not have been worse," Merritt said, adding that he fears the rumors could make the government shy away from the controversial marketing scheme.

Despite the dogged support of Kenya's top leaders, anthropologists and family-planning specialists say this country remains extraordinarily uncomfortable with birth control. They say large families (average family size here is eight) continue to be one of the most important ways in which Kenyan couples measure their wealth, as well as their moral rectitude. For men in Kenya, according to family-planning specialists here, fathering children continues to be society's most important measure of virility.

On top of this cultural bias in favor of large families, the influential Roman Catholic Church in Kenya has taken a strong stand against the government's family-planning programs. The Catholic Church, representing about 26 percent of the country's population, has accused family-planning agencies associated with the government here of aiming their programs at children without regard for family or Christian values.

Rumors over contraceptives in school milk and in immunization shots are but the most recent in a long line of false word-of-mouth birth-control alerts that have spread across Kenya in recent years.

The U.S. government, the usual suspect in purported plots to sneak infertility drugs into food here, last year was believed to have doctored its donations of yellow corn for famine relief. The government had a difficult time getting even hungry people to accept the yellow corn. Kenya Breweries recently had to destroy tens of thousands of bottles of beer when a new bottle design was rumored to signal the advent of no-children beer.

At the Mugumoini Primary School here in Thika, a deputy headmaster said that parents have been convinced that it is safe for their children to drink school milk.