U.S. Postmaster General William F. Bolger said today that Saturday mail delivery probably will be dropped in order to meet the Carter administration's new streamlined inflation-fighting budget.

"As undesirable as this may be, the Postal Service may have no other choice," Bolger said.

Bolger told the U.S. Postal Service board of governors that the Postal Service had three choices on how to trim its budget: increase productivity, raise postal rates or cut mail delivery from six days to five.

The Postal Service already was planning to increase productivity by 3 percent, and "an increase beyond this would be practically impossible to achieve," Bolger said.

An increase in postal rates would be inflationary and "would defeat the purpose of the cut, which is to help balance the federal budget as a means of stemming inflation," he added.

However, the board of governors has been considering postal rate increases that were planned before the budget cuts were announced. No decision has been reached on the new rates, which are expected to take effect next year, Postal Service spokesmen said.

"This appears to leave only one workable alternative, namely effecting a service adjustment; and the only service cut that could possibly achieve savings at the level being discussed is elimination of a sixth day of delivery," Bolger said.

Prior to Carter's new budget cuts, Bolger had said that elimination of Saturday service would be unnecessary. Bolger said today that the issue will be studied during the next few weeks before any final decision is made.

Postal service studies showed that elimination of one day of mail delivery could save between $500 million and $600 million a year, Bolger said. During the first year, however, only about half of that amount would be saved, he said.

Bolger added that $35 million in fuel costs could be saved by switching to five days of mail delivery. Other services still would be available on Saturdays, a Postal spokesman said.

Carter this week sent Congress $17.2 billion worth of proposed spending cuts for fiscal 1981 that are intended to help curb inflation. The House Budget Committee voted last week to reduce fiscal 1981 postal appropriations of $1.525 billion by $836 million.

The Office of Management and Budget proposed this week that those appropriations be reduced by $250 million in fiscal 1981, by $644 million in fiscal 1982 and by $552 million in fiscal 1983, Bolger said.

In 1977 the Postal Service experimented in 16 cities to determine the effect of five-day mail delivery. But the Post Office handled about 8 million fewer pieces than it does now, Bolger said.

"Existing studies on these matters are limited and dated," Bolger continued, "and they do not tell us which customers would be most affected by such a move and whether dropping Saturday delivery as opposed to another day would be least disruptive."

Bolger said that two task forces will be named to study the effect this plan would have on mail service and on labor relations.

If the Postal Service decides to eliminate one day of delivery, a proposal must be submitted to the independent Postal Rate Commission, which will hold hearings and issue an advisory opinion. This could take many months.