South Korea's positive response to the latest unification overture from the North means that the two countries are closer to serious negotiations than at any time in the last eight years.

The new connection could be switched off quickly by either side, returning the divided peninsula to the stalemate that has existed for nearly 35 years. But South Korean sources sense that the North, for reasons that are obscure, is suddenly interested in serious talks on reunification.

In the past, offers to talk have been largely written off as propaganda exercises by the other side. They have been couched in impossible terms certain to lead to rejection and designed essentially for domestic political purposes -- to show people on both sides that the other government was the obstinate one.

This time, the South could not refuse the North's offer without appearing to be obstructive. The North offered to hold talks between "authorities" of both governments, abandoning its old insistence on arranging meetings only between private groups and political parties.

The late South Korean president, Park Chung Hee, had offered talks between responsible "authorities" a year ago. The North countered with its idea of private-party conferences, and the whole affair died a slow death after unproductive talks last February in the truce village of Panmunjom. In promising a "positive" response in Seoul today, President Choi Kyu Hah emphasized that the North is now using Seoul's language.

There is one hesitancy in Seoul. Officials there believe the North may be acting generously now only because it perceives a South Korea weakened by Park's assassination on Oct. 26 and by instability in the military ever since.

North Korea's peace offensive began almost as soon as Park was buried with an offer to establish a joint sports team for the Olympics. That was followed with an offer to revive the Seoul-Pyongyang telephone hot line. To many South Koreans and some Americans, the offers were designed to take advantage of the South's troubles and win what it could.

What puzzles analysts in Seoul is the evident haste of the North's latest maneuverings. One incident in particular is noted as an example of the North's apparent sense of urgency to get things under way.

Last Saturday, after North Korea declared it wanted to send unification proposals to Seoul, South Korea at first demurred, demanding to know to whom the messages were directed. Almost immediately, the North responded with a clarification and the letters were delivered at Panmunjom in the Demilitarized Zone, according to an account given by South Korean officials.

From that and other details, Seoul detects a sense of urgency in the communist government of Kim Il Sung.

The letters proposing talks also spoke of a kind of emergency situation. One of them said: ". . . We now find ourselves at a crucial juncture, where the very complicated situation around our country is sounding time and again an alarm bell urging us to reject the outside forces and pave the way for reunification without delay. . ."

South Korean officials speculate that something brewing in the international arena, something threatening to the North, lies behind the use of language about "alarm bells" and the like.

One source in Seoul suggested that the Soviet Union's toppling of a communist government in Afghanistan may have sent chills running through Pyongyang. North Korea has been nominally allied with both communist giants, the Soviet Union and China, although in fact it has tilted ever closer to China in the past 18 months.

American officials have long thought that the North wanted to escape its isolation and dependency. China is believed to have encouraged Pyongyang away from Moscow and to have endorsed the idea of trying to settle things with South Korea in some fashion. Peking was quick to back North Korea's latest offer last weekend, which came at a time when U.S. Defense Secretary Harold Brown was visiting China to promote military cooperation. The Korean issue was discussed between Brown and the Chinese, officials have said, but no details of those talks have been made public.

The first meeting between North and South is likely to be between South Korean Prime Minister Shin Hyun Hwack and his northern counterpart, Li Jong Ok, who issued the invitation. The North said they could meet at Pyongyang, Seoul, Panmunjom or in some third country. Indonesia, which has embassies in both countries, might be a possible site.

Moreover, the North's proposal held out the possibility of an early meeting between heads of state, which would mean Choi representing the South and Kim Il Sung the North. Pyongyang's offer mentioned a desire to "bring to maturity talks between the high-level authorities."

North Korea has not abandoned totally its hope of having separate talks among private groups from both countries in what it calls a "comprehensive political consultative conference." This time, however, the letters made clear that the central issue is talks between government officials, which is what the South has always wanted.