BEJING — Beijing. -- Last Saturday night, U.S. officials who traveled here with Secretary of State James A. Baker III were portraying Sino-American relations as having reached a critical juncture.
The day before, Mr. Baker, by coming to Beijing to talk with senior Chinese leaders, had delivered a symbolic concession long sought by China. But with only one meeting left, the Chinese leadership appeared unyielding in response to U.S. complaints over China's human-rights abuses, wanton arms sales and trade malpractices.
Then, last Sunday, events seemed suddenly to change course at the last minute. Mr. Baker's final meeting with China's Foreign Minister Qian Qichen, scheduled to last only 45 minutes, stretched to more than five hours. When it finally ended, Mr. Baker announced that he had achieved ''some clear gains'' in the areas of arms control and trade and some lesser improvements in the field of human rights.
But the aura of a dramatic turn-around in the talks only obscures uncertainty over what really was achieved. If the U.S.-Chinese relationship was at crossroads last Saturday night, it appears to remain so after Mr. Baker's departure from Beijing.
Perhaps evidence will emerge sometime later -- after a necessary, face-saving interval for China -- that the strenuous negotiations resulted in substantive changes in China's hard-line positions. President Bush last week hinted at that by saying, ''When it's all out, I think people will think some progress is made.''
But for now, the agreements announced by Mr. Baker generally fall into several dubious types: Promises that China had made prior to his trip, those that were already in the works by virtue of existing American leverage and those about which there is some doubt that China will deliver. Some examples:
* China told Japan in August that it would sign the Nuclear Non-proliferation Treaty. Then it dragged its feet, in an apparent effort to set the stage for promising Mr. Baker that it will complete all of the treaty's formalities by the end of March 1992. Meanwhile, allegations have continued to crop up that China has been transferring nuclear technology to Middle Eastern nations.
* China made ''positive proposals'' to resolve U.S. complaints over the widespread pirating of American copyrights and patents here, Mr. Baker said. But China's violations of intellectual property rights are already the subject of a U.S. trade investigation that ends Tuesday. If China does not come up with substantive steps by then, the U.S. within a month is supposed to enact punitive tariffs that would cost China perhaps a billion dollars or more annually.
* China intends ''to observe'' the Missile Technology Control Regime, Mr. Baker said, if the U.S. drops its sanctions against the sale of advanced computers and satellite technology to China. ''To us, this means that they will apply [the regime's guidelines] to any exports of missiles and related technology,'' he said, including its planned sales of M-9 missiles to Syria and M-11 missiles to Pakistan.
But China reportedly had already committed to abiding by the regime in private meetings with American diplomats earlier this year -- without attaching any conditions. Moreover, China from time to time has maintained that many of its missile exports would not come under the regime's guidelines because the weapons' ranges are too short.
And although a Chinese foreign ministry spokesman said Thursday that ''China will honor its word,'' there is much evidence that China's leaders are not entirely in control of the nation's free-wheeling weapons dealers.
* China finally gave the U.S. an accounting for many of the more than 800 names on a U.S. list of suspected political prisoners still held in connection with the 1989 Tiananmen Square protests. Such an accounting has been generally demanded by the U.S. for more than two years, and lists of suspected jailed dissidents were submitted earlier this year by both the U.S. and Germany.
There is no indication whether China's response to the U.S. list is credible. A Chinese foreign ministry spokesman Thursday raised doubts about that by labeling such foreign-generated lists as ''not serious,'' dismissing them as based on hearsay and claiming that some of the listed persons do not exist.
There also is no indication whether China's responding to the list could be the first step toward the release of more political prisoners. If China takes that step, though, it is likely to do so only in order to sway the ongoing U.S. congressional debate over renewing next year its profitable, most- favored-nation trading status with America.