Vietnam relented tonight and agreed to receive a delegation of American congressmen in Hanoi to discuss policies on refugees who have fled that country in large numbers, American sources said.

There was no explanation of why the government suddenly reversed itself and decided to admit the delegation after first saying it would not be received as scheduled.

Hanoi radio had said early today thaat the congressmen would not be accepted because of critical remarks made last Saturday by one of them, Rep. Robert Drinan (D-Mass.).

The reversal surprised many officials and congressmen who had concluded that appeals to Hanoi were useless.

The congressmen were retiring for the night in their hotel here, uncertain of whether they would fly to Hanoi Wednesday of head back to Washington with the most important part of their six-day mission unfulfilled.

The word came in a telephone call from the U.S. ambassador to Thailand, Morton Abramowitz, whose ambassy participated in making strong appeals for Hanoi to change its mind. No explanation was given, but sources here said the Vietnamese government would make a statement later saying Drinan would be allowed to make the trip with the rest of the delegation.

Rep. Benjamin Rosenthal (D-N-Y.), the delegation chairman, said, "I am very pleased because it shows that they acted in a mature and responsible way consistent with improving relations between our two governments."

Drinan was truimphat and said the reversal shows the Vietnamese had recognized "the folly of their mistake."

"When reason is pointed out to Hanoi, they do in fact submit," Drinan added. "Perhaps this is a good sign. They have seen the folly of their mistake in trying to ban a group of congressmen and 17 journslists, and maybe this same credibility, this same rationality will prevail when we tell them they cannot send out thousands of people on boats to just disappear from their country."

Drinan, a Roman Catholic priest, was an early and vociferous critic of the U.s. involvment in the Vietnam war and of the former government of South Vietnam. He supported House efforts to stop the U.S. bombing of North Vietnam and to cut off financial support of the war effort.

Last Saturday in Hong Kong Drinan denounced Hanoi policy toward dissenters, saying it had established "ner gulags" and "concentration camps" in which to detain those who disagreed with Communist policy.

Radio Hanoi responded this morning by blasting Drinan for making "slanderous" statements about Vietnam policy on reuniting Vietnamese with their families living in the United States. The broadcast said the delegation would not be received "as scheduled."

During the day, pessimism increased as the Vietnamese embassy here said the congressmen would not be welcome in Hanoi.

Appeals were made through U.S. embassies to Vietnamese ambassadors in Bangkok and Paris. Assistant Secretary of State Richard Holbroode, speaking for Secretary Cyrus Vance asked Hanoi to reconsider. In a message to the Vietnamese Foreign Ministry, Holbrooke said the committee's findings could influence American policy.

The State Department also informed the Vietnamese that such a rejection would be considered an affront to Congress and asked Hanoi to understand that the delegation includes members who hold a wide variety of views, according to American sources.

Exactly what the congressmen will discuss with officials in Hanoi remains a mystery. Rosenthal said tonight his delegation will "just sit and listen" for the first two hours, without proposing anything.

The delegation spent much of the day attempting to soften the impact of Drinan's remarks by making conciliatory commnnts they hoped would be noted in Hanoi.

The most pointed effort to disassociate the other members from Drinan's remarks was made by the ranking Republican, John Paul Hannerschmidt of Arkansas. He said Drinan's views were not necessarily those of other members.

Drinan was trying to have it both ways.

"I'm sorry if they misunderstood this," he said at one point. But later he refused to back down and again accused the Vietnamese government of keeping hundreds of thousands of people in detention.

Several committee members, although refusing to critize him publicly, said privately that Drinan had committed an error in his strident remarks, and agreed that Vietnam had reason to be angry enough to withdraw the invitation.

In an interview, Drinan said he spoke out in Hong Kong after several ethnic Chinese refugees there had told him that dissenters in Vietnem were treatened with confinement to the "new economic zones." He said they insisted that many people sent there "never come back."

The "new economic zones" are barren areas of Vietnam where former city dwellers, most of them Chinese have been sent after their property was expropriated. They are expected to take up farming and become self-supporting. Many refugees have said they fled the zones to escape by boat because they feared they would starve.