Energy Secretary James R. Schlesinger said yesterday that if Iran's oil exports remain shut off for another three months, Americans will face possible gasoline rationing, price increases and government allocation of fuel supplies.

He also declared: "I think we have to recognize now that Iranian production is unlikely ever to return to the pre-crisis level.

Before the current political turmoil, which has undermined the power of Shah Mohammad Roza Pahlavi, Iran was producing about 6 million barrels a day.

Schlesinger noted that foreign oil technicians have left the country and that "discipline in the oil fields is low. He said: "It will take time simply to bring production back to 3.5 or 4 million barrels," and added that if output eventually returned to between 5 and 5.5 million barrels a day, "I think that would be fortunate.'

He made the pessimistic forecasts in an interview on "Face the Nation" (CBS, WDVM).

It was the first time he had said that government-imposed allocations, rationing and higher prices are "possibilities" as a result of Iran's oil shutdown. Last weck he said the United States would have to use stringent conservation measures if the shutoff continued into the fall, but he termed gasoline rationing unlikely.

Yesterday he repeated his earlier comments that this country can offset the 500,000 barrels a day of oil it is no longer able to import from Iran by voluntaty savings, adhering to the 55mph speed limit, setting thermostats at 65 degrees Fahrenheit and cutting out unnecessary driving.

But he warned that "we are drawing down [oil] stocks worldwide by something on the order of 2 million barrels a day in excess of normal. We are borrowing against the future. If this shutoff were to go for three months, we would have to begin steps to constrain demand here in the United States."

The secretary added, "I think we should begin to plan for increased production elsewhere." He specifically mentioned California and the North Slope of Alaska. Current Alaskan poduction has allowed the United States to cut its imports from 8.8 million barrels a day in 1977 to 8 million last year, he said.

Other countries have increased their output to compensate for the loss of Iranian oil. Saudi Arabia, for instance, has pushed its production from 8.5 million to 10.5 million barrels a day.

Schlesinger's view of Iran's capacity to rebuild its oil output is disputed by some international oil experts. They say that no matter who is in power in Iran, its leaders will continue to look to oil sales as the primary source of foreign revenue. Iran's oil revenue now exceeds $20 billion a year.

The experts also point to Libya, which underwent a coup in 1969 when Muammar Qaddafi ousted the royal government. Oil production was nationalized and initially dropped dramatically, but later returned to normal, these sources say.

Iran can turn to oil technicians from Europe and Arab nations to compensate for its loss of U.S. expertise, the sources say. Americans are the largest group of foreigners forced to leave Iran by recent violence.

Schlesinger predicted that U.S. gasoline prices would rise "6 or 7 cents" by the end of this year because of the loss of Iranian oil and the 14.5 percent increase in world oil prices announced recently by the Organization of Petroleum Exporting Countries.

Most of the increase in U.S. prices will result from the OPEC action, he said.

Asked what impact the Iranian situation is having on administration plans to remove price controls from domestic crude oil production, Schlesinger repeated an earlier statement that the administration will delay the proposal. He said the Iranian cutoff, the OPEC announcement and fears of inflation have all "militated against decontrol" any time soon.