If, as Ted Koppel described it, "Nightline's" presentation of a full week of programs from South Africa was "unprecedented television" for the United States, then for South Africans the intrusion of American-style interaction into their own segmented society was an experience in cultural shock treatment.

For some reason known only to the enigmatic minds in Pretoria who serve as the ruling Afrikaners' cultural guardians, a decision was made to show the ABC News programs on South Africa's own semiofficial television service.

The result has been extraordinary. The on-screen questioning of cabinet ministers and the argumentative exchanges between spokesmen for the apartheid government and some of their African nationalist opponents are things South Africans have not seen before. The on-air debate has angered some and left others charged with excitement. Among all, it has made the "Nightline" programs the conversation piece of the year.

"Imagine if this idea of having people in this country talk to one another caught on," Koppel said. "You would never put the genie back in the bottle. The interaction of ideas can be very contagious."

Koppel wanted to bring the program to South Africa because he thought the recent surge in U.S. interest in apartheid made it important to give Americans a background against which to follow the news.

"There are 10 to 12 million Americans who have learned more about the complexities of South Africa over the last three nights and who can now layer today's story over that context," he said in a conversation on the day the South African police killed 18 demonstrators in the eastern Cape Province city of Uitenhage.

Yet the importation of the American habit of vigorous democratic debate to this troubled society, as compartmentalized in its ideas as it is in its racial structure, is surely even more important. And Koppel believes the screening of the programs in South Africa may do some good.

In politics as in technology, Koppel noted, Americans are conditioned to regarding the miraculous as commonplace. There is nothing exceptional to the viewers of "Nightline" -- who have seen confrontations between the representatives of Iraq and Iran, Israel and the PLO, Britain and Argentina at the height of the Falklands war -- about now seeing Nobel laureate Bishop Desmond Tutu and Foreign Minister R.F. (Pik) Botha debating 10,000 miles away.

But to on-the-spot South Africans, it is a kind of interaction they have not witnessed before. South Africa has a relatively free press by the standards of the African continent and even beyond, but broadcasting has always been tightly controlled.

Television was prohibited until 1975. When it was finally permitted, it was carefully structured on apartheid lines with separate channels for blacks in their tribal languages, which few whites can understand. When one of the nominally independent black "homelands" started its own TV service in English last year, Pretoria spent a fortune fixing the transmission so that it was beamed only to members of that particular black tribe, with minimum "spillage" into white areas.

The result is an almost total lack of political interaction on the screen. The few blacks who appear are usually quiescent participants in the apartheid system, and even they are given little time. In the past, Tutu's rare appearances have usually presented him in an unfavorable light. Other African nationalist opponents of the government do not appear at all.

Cabinet ministers are never subjected to the kind of forthright questioning that is standard fare on American television. That happens at press conferences, but not in TV or radio interviews. For those, tame interviewers ask questions that launch government spokesmen into lengthy monologues.

For Koppel, busting in on a society brought up on this kind of television culture has been as novel and exciting an experience as watching his programs has been for the South African viewers.

He has thrown together Tutu and Botha, white far-rightists, middle-of-the-roaders and black radicals. He has juxtaposed on screen individuals who have never exchanged views before and would not dream of meeting personally.

Back home it may be good intstructional stuff, but here in South Africa it is unique. Koppel can walk the streets of Johannesburg the next day and get a playback on the impression he's made.

"I met an Afrikaner the other day," he said, "the sort of guy one would have expected to be really hard-line, and he told me how thought-provoking he had found it hearing Bishop Tutu speak about not being able to vote although he is an educated, responsible man of 53, a bishop of his church who has lived here all his life. He said he had never thought of it that way before."

But the walls of apartheid are not those of Jericho to be brought down by a trumpet blast, even when amplified by the power of television. The survival imperative of the outnumbered Afrikaners on which all white South African politics is predicated, encourages a more impervious spirit than that.

Still, "Nightline" has undoubtedly contributed to the slow weathering of those walls. So why did Pretoria permit it?

There were clear advantages in having the programs broadcast in the United States. South Africa believes it is the victim of a biased international press and it has spent millions, both clandestinely and above ground, trying to put across a viewpoint that it is convinced a civilized world would understand if only it can receive it clear and uninterrupted.

Although the suspicion of television is deep-seated, "Nightline," with its reputation for impartiality, offers the opportunity for getting across a viewpoint. When Richard Kaplan, "Nightline's" executive producer, put this point to Ambassador Bernardus Fourie last year, the South African envoy in Washington was quick to respond. He had appeared on the program two years ago and felt he had received a fair deal.

Fourie talked his government into cooperating, and visas were issued to the "Nightline" team.

That did not mean all was cut and dried, however. When the advance team arrived in South Africa, it immediately ran into problems. As Koppel said, "It's one thing to talk about interaction in a general sense, another when you start putting names to it."

When the idea of a debate with Tutu was put to Pik Botha, he was incredulous. "You want what?" he asked. "We don't do that sort of thing here."

But after another week of persuasion, with reassurances from Fourie, Botha agreed. It was tough going, but Koppel is full of praise for the way the deal has been honored. "They have been true to their word," he said. "They have put no obstacles in our way."

Understanding why South Africa decided to show the programs locally is more problematic. There are indications that South African leaders believe they made a misjudgment. After the first two debates they began backtracking. Constitutional Affairs Minister Chris Heunis pulled out, President Pieter W. Botha threatened to do likewise, and the local broadcasting service cut Winnie Mandela, restricted wife of imprisoned black underground leader Nelson Mandela, from the show.

There were problems on the black side as well. Advance reports that the government was cooperating with the "Nightline" team were enough to damn them in the eyes of black radicals. But in the end the reconciliatory Tutu, who has a sense of publicity as finely honed as his sense of justice, played counterpoint to Fourie and persuaded the black radicals to participate, too.

Fascinating yes, enjoyable no. Walking a tightrope between these two grievously alienated sides was too much of a strain to be fun, said Koppel.

For once the anchor may have found it difficult to maintain his professional impartiality in what is perhaps the world's most emotionally challenging society.

Warming his bones in the southern summer sun, he gazed skyward on the day of the Uitenhage shootings and softly recited John F. Kennedy's favorite quote from Dante: "The hottest places in Hell are for those who preserve their neutrality in a crisis."

"This business of trying to keep your distance from both sides is good journalism," he added, "but it's very tough on the human person."