Two Cuban doctors, allegedly being flown home under guard after they tried to defect in Zimbabwe, managed to thwart their captors Friday when they slipped a note pleading for help to an Air France official at the Johannesburg airport.

"KIDNAPPEDS," said an underlined word in the margin of the three-page, handwritten note.

In a scenario worthy of a Cold War spy novel, the doctors wrote they were being flown to Cuba under guard after being abducted by armed Zimbabwean police working with Cuban diplomats. "We are traveling, kidnapped, to Cuba," said the note, written in passable English and handed to the airline official while the group was in transit in South Africa. "We are closed to departure to Paris, where we try to do something. Please. We are very concern about our lifes and the well being of our family in Cuba."

Air France refused to allow the two doctors and accompanying security guards, whose nationality was not clear from the note, to board its flight from Johannesburg to Paris, and South African authorities sent the group back to Harare, the Zimbabwean capital. The doctors, members of a Cuban aid mission to Zimbabwe, have not been seen since.

The two physicians, Leonel Cordova Rodriguez, 31, and Noris Pena Martinez, 25, disappeared nine days after they publicly denounced Cuba's Communist government and sought asylum at the Canadian embassy in Zimbabwe.

The office of the U.N. High Commissioner for Refugees, which had scheduled a meeting with the two in Harare on the day of their disappearance, sent a letter to the Zimbabwean government Monday insisting that they be produced. Kris Janowski, spokesman at UNHCR headquarters in Geneva, said yesterday that no response had been received.

Western diplomats in Harare said they believe the doctors were being held in Harare Central Prison. Both the diplomats and Janowski confirmed the basic outline of the alleged kidnapping, which was reported in the Miami Herald yesterday. "Something happened there," Janowski said. "They did try to take them somewhere. . . . It's very difficult to get it straight at this stage, [but] something was afoot."

George Charamba, spokesman for Zimbabwean President Robert Mugabe, did not return telephone calls seeking comment on the allegations. A spokesman for the Cuban Interests Section in Washington said it had no information on the matter.

The kidnapping allegations appear to tarnish Cuba's reputation just as it has been enjoying an international public opinion bonanza from the Elian Gonzalez case in the United States, and as pressure has been growing in Congress for at least a partial lifting of the U.S. trade embargo on the island nation.

The reported incident also seems to cast a shadow over Cuba's highly praised program of "doctor diplomacy," in which thousands of Cuban physicians are sent to deliver free health care to poor countries.

Cuba's health care system is widely cited as one of President Fidel Castro's foremost domestic accomplishments. All medical care on the island is free; indicators such as infant mortality and life expectancy are nearly on a par with those in the developed world and are far better than in most of the rest of Latin America. Its ratio of 53 physicians for every 100,000 people is nearly double that of the United States, according to the Pan American Health Organization.

So much emphasis has been placed on medical training that the island has an overabundance of training facilities and physicians. As a result, the government invites medical students from throughout Latin America and the Third World to study medicine there. At the Ibero-American summit in Havana last November, Castro inaugurated an 8,000-student Latin American School of Medicine and invited Latin and Caribbean governments to send students free. He promised that they would not "be taught politics, as is the case with Cuban students in our universities," and that government transportation would take them to their church of choice each week.

Excess doctors are sent overseas for tours of aid duty in poor countries or to work temporarily in disaster areas. The policy has led to some celebrated defections, including those of three doctors and a nurse who left an aid mission in Nicaragua for asylum in Costa Rica in 1990, and of three who were part of an emergency assistance team sent to Venezuela early this year to aid flood victims. Like Zimbabwe, Venezuela has good relations with Cuba and at first denied them asylum. Later, though, Venezuela said it would grant them a one-year "transitory" stay for "humanitarian reasons."

In the latest case, the two doctors arrived in Zimbabwe in April as part of a team of 152 Cuban physicians offered to Mugabe to help relieve staffing shortages in rural government hospitals when he visited Havana last September. Canadian officials confirmed that they came to the embassy on May 24 and requested asylum. Following normal procedure, the Canadians--who have no asylum officials in Harare--sent them to the local UNHCR office for consideration of their case. If UNHCR declared them legitimate refugees, they could ask to go to Canada.

That same day, in an interview with the Daily News, a Harare paper, Cordova said: "We want to go to Canada and work there if possible. We were sent here under the policies of Fidel Castro so that he can appear to the world as a good man. He sent us here for his political goals. I want to live out of Cuba."

Cordova, who said he has a wife and three children in Cuba, complained that the Cuban government was not allocating enough money for them to live in Zimbabwe. Pena said that doctors in Cuba are paid only about $25 a month. The two said they decided while still in Cuba to defect to Canada once they arrived in Zimbabwe. The newspaper quoted the Cuban ambassador in Harare, Rudolfo Sarracino, as saying the two "willingly decided to come here. They are not persecuted by the Cuban government, and there is no reason why they should politicize the issue."

According to the UNHCR's Janowski, the two were given an interview appointment for last Friday and sent to Zimbabwean asylum authorities, who gave them an appointment for the same day. "They were sent by Zimbabwe authorities to some sort of accommodation for asylum seekers, which they apparently didn't like," he said. "They moved in with friends" and never showed up for the Friday interviews.

According to the note passed to Air France, written on paper with a South African Police Service letterhead, "at 4:17 a.m. the police of Zimbabwe, 2 officer with a gun machine arrive to the place we were in Harare waiting for the process of Political Asylum seeking." They were put into a jeep and taken to a Zimbabwean immigration facility.

There, the note said, they refused demands to sign papers and be fingerprinted. "In the building was the Consular Officer of the Cuban Embassy." Later, they were taken to the airport, the note said, where they saw Cuban ambassador Sarracino, another Cuban diplomat and the head of the Cuban aid mission. They communicated their situation to the South African flight crew, the note said, and South African police were waiting for them in Johannesburg. But the South Africans could not help "because we were in transit," the note said.

According to the Miami Herald, upon arriving in Paris an Air France crew member faxed a copy of the note to the UNHCR in Geneva and hand-delivered another copy to his neighbor, a Cuban American living there.

Janowski said Cuban defection cases are "extremely, extremely rare" for UNHCR, since "most Cubans escape to the U.S.," where they are nearly always automatically eligible for resident status. Asked whether the two would likely qualify for international refugee status and asylum, he said it is "difficult to speculate on what would happen. But, of course, the fact is people who leave [Cuba] and denounce the government are unlikely to be greeted back with flowers."

Correspondent Jon Jeter in Johannesburg contributed to this report.