"Nuclear War: The Incurable Disease," at 10 tonight on Channel 26, is fascinating, not so much for its intrinsic worth as television fare, but for where it has been and who has been watching it.

It's a Soviet production that was seen, it is believed, by about 200 million Soviet citizens last summer. It is a friendly, even warm, discussion between American and Soviet physicians on the subject of the nuclear arms race, on the medical consequences of nuclear war and of the desires of the medical professionals of both powers to see an end to what they call the "world's number one public health problem."

The panel discussion was put together at an international cardiology conference in Moscow by members of the International Physicians for the Prevention of Nuclear War (IPPNW), the sibling group to Dr. Helen Caldicott's Physicians for Social Responsibility. Three American and three Russian cardiologists sat at a round table and talked peaceably about Armageddon.

It was the first time in anyone's memory that Russians and Americans engaged in a televised dialogue in the Soviet Union on any subject whatsoever.

Its reception was so huge that it was rebroadcast a few days after its first airing last summer and subsequently has been shown in Soviet-bloc nations, according to Joan Gilmour, spokeswoman from "Inside Story," the independent public affairs series under whose auspices the film is being rebroadcast.

Its message is essentially the one American physicians have been communicating for some months now: that there is no winner in a nuclear war, that 80 percent of medical professionals and virtually all hospitals and drug warehouses will go in the first blast. The agonizing films from Hiroshima and Nagasaki again underscore the message. Basically it's a short course in "The Final Epidemic."

But it is also Russians and Americans on Russian TV agreeing that there is no point to civil defense programs, which are much larger pet government projects in the U.S.S.R. than in the United States. It's Russians and Americans agreeing that, as Harvard cardiologist and IPPNW president Bernard Lown put it, "When you're at the edge of an abyss, progress is not going forward. It's stopping."

What, you may ask, is the catch? Why did the Russians let them do it?

"Inside Story" and its chief correspondent, Hodding Carter, are not about to let us go wide-eyed into the devious motives of the Kremlin. A post-film dicussion with Carter, ABC Soviet correspondent Anne Garrels and Cable News Network's Stuart Loory sheds some light.

The film fits nicely into Soviet propaganda: The United States dropped the bombs at Hiroshima and Nagasaki and therefore started the arms race, goes the Kremlin line, and, moreover, is the most likely to start a nuclear war today.

The program's popularity stemmed, suggests Garrels, from the terror the Russian people have of "the Reagan rhetoric," and from their hunger to see what real Americans are really thinking.

Unhappily, the Russian hunger for the Western point of view apparently is not reciprocated in the United States. At least, said Gilmour, the major networks showed no interest in airing this exercise in international accord even as an extraordinary piece of television history.