Passenger service by National Airlines and Pan American World Airways returned to normal late yesterday with the end of a two-day walkout by union members that had disrupted almost all of National's domestic flights.

Merle Richman, spokesman for the recently merged airlines, said National had resumed its regular flight schedule by 5 p.m. yesterday. Earlier yesterday only two of nine flights departed from Washington National Airport and early afternoon flights to New York and Florida also were canceled.

All but one of Pan Am's 179 scheduled flights were expected to depart yesterday, according to Richman.

'the nationwide walkout forced cancellation of 122 of National's 135 daily flights on Thursday, including 13 of 16 flights leaving from National Airport. Pan Am, which merged with National on Jan. 19, had canceled five flights Thursday.

The National Airlines ticket counter at National Airport had the look of hectic normality at midday yesterday. Passengers with reservations on National flights had been called overnight and offered seats on other airlines. These substitutions prevented a repeat of Thursday' harried airport scenes, officials said.

Richman said many of the morning flight cancellations at National Airport had been planned in advance because of uncertainty over whether the employes -- 7,500 nationwide -- would comply with a back-to-work order issued Wednesday by U.S. District Court Judge Henry Bramwell in Brooklyn.

In addition, planes grounded by the strike were "scattered and had to be put back into position" before the full flight schedule could be resumed, Richman said.

The strike began on Wednesday when members of the Transport Workers Union walked out to protest Pan Am's dismissal of 16 workers on the West Coast.

Baggage handlers and ground crews were involved in the strike, but the crippling blow to National's operations was struck by the airline's 41 dispatchers in Miami who are responsible for monitoring all flights from takeoff to landing.

"Thirty to 40 peope really had all of National tied up," said Richman. "Management people could cover most of the Pan Am flights, but they couldn't cover all of National's.