The Salisbury government of Prime Minister Abel Muzorewa received a badly needed boost today when Rev. Ndabaningi Sithole and 11 collegues ended their boycott of Zimbabwe-Rhodesia's first black-dominated National Assembly and took their seats in the 100-member body.

Until now, Sithole, a major rival of Muzorewa, has refused to participate in the government because, he charged, the country's first universal-suffrage elections last April were tainted with "gross irregularities." He made the accusation after his Zimbabwe African National Union (ZANU) was roundly defeated at the polls by Muzorewa's United African National Congress.

His criticisms of the government and the electoral process undermined the credibility of Muzorewa's government of national unity, which already was under siege by guerrilla forces of the Patriotic Front. Neither did Sithole's charges help to dispel international skepticism about the stability of Muzorewa's biracial government.

Sithole's turnabout comes just as the Salisbury government is facing fierce attacks from black states represented at the British Commonwealth conference in neighboring Zambia. The other African states, in particular Nigeria, have warned Britain not to recognize the Muzorewa government in the former British colony.

Although Sithole today told reporters he would work "with and for the government," some political observers doubt that he has abandoned entirely his stated intention to work for the collapse of the Muzorewa administration and the staging of a new election under international supervision.

Sithole also said although his ZANU party would take up the two Cabinet positions it is entitled to hold under the new constitution, he himself would not occupy either post, but would devote himself to party matters.

Several factors appear to have led Sithole to end his boycott of the National Assembly, not the least of which may have been the killing of 183 armed soldiers loyal to his party in a clash with government security forces July 20.

Official accounts said the ZANU "auxiliaries" who were attached to the government defense forces, were fired upon when they refused to obey an order from army commanders to report for a "reorientation" course. Instead, in Sithole's name, they reportedly tried to establish a military hold over a certain area of the country.

Since that incident, hundreds of ZANU "auxiliaries", some of whom are former guerrillas who changed sides to join Sithole, have been detained by the security forces, according to ZANU. ZANU spokesman James Dzvova has said some of them are in detention.

From the outset of Sithole's campaign against the Muzorewa government, police periodically have detained many ZANU officials and have made several raids on ZANU officers, as well as on Sithole's home, in what they said were searches for arms.

"These were warnings that the government would hit hard at any extraconstitutional actions by ZANU," said one political observer in Salisbury. "This no doubt led to a rethink on tactics on ZANU's part."

In addition, there reportedly was dissatisfaction within ZANU over Sithole's decision to boycott the National Assembly, and it appears that pressure from within his own party -- expecially from the elected delegates -- was another factor that led him to drop his objections to their participating in the assembly.

"It was becoming obvious that continued boycott was not achieving anything," one analyst said.

Finally, under the constitution, the 12 ZANU delegates would have had to forfeit their assembly seats if they did not take them up during the first 21 days of the parlimentary session. The deadline is Aug. 14.

Sithole apparently decided not to await a decision on a petition asking the high court to declare the April elections null and void. There is little chance the petition. to be heard Aug. 29, will be granted.

A Muzorewa party spokesman welcomed ZANU's decision, saying it was "better late than never." The Muzorewa party holds the largest bloc in the assembly, with 44 seats.