Consumer prices fell 0.3 percent in April, marking the steepest three-month price decline in 37 years, the Labor Department reported yesterday.

The string of rare monthly declines, caused almost entirely by falling oil prices, makes it all but certain that no cost-of-living adjustments will be made next January in Social Security and other federal benefit programs without new congressional action. For those COLAs to be paid automatically, prices must rise at least 3 percent between the third quarter of 1985 and the third quarter of this year. Forecasters expect prices to be up only about 2 percent over that time.

The dramatic decline in oil prices, and the temporary drop in the overall consumer price index, are having other economic effects. Some analysts say the highly visible price drop has contributed to smaller wage gains this year. For example, the Labor Department's hourly earnings index rose only 2.5 percent in the past 12 months, the smallest rise since the numbers were first calculated in 1965.

So far this year, prices as measured by the consumer price index have fallen at a 2.3 percent annual rate. The rate of decline in the CPI from February through April -- 4.3 percent -- was the sharpest since the three-month period that ended in January 1949, when surplus crops depressed food prices, the department said.

Oil prices have begun to stabilize, and gasoline prices have crept up in the past month, suggesting that there may not be future price reductions as a result of cheaper oil.

Economists expect consumer prices to fall for perhaps another month, and then to accelerate to a rate between 3.5 and 4 percent. But few economists anticipate a return to rampant inflation.

The inflation report came on the heels of other good economic news this week, an upward revision in the government's estimate of growth in gross national product from 3.2 percent to 3.7 percent for the first quarter.

White House spokesman Larry Speakes said the recent economic reports help "quantify what millions of Americans have been experiencing for 41 consecutive months: They're enjoying an economic expansion that shows no signs of diminishing."

Economists said that they expect oil prices to stabilize between $15 and $20 a barrel for the rest of this year and most of 1987. After hovering around $15 for several weeks, the price of West Texas Intermediate -- the benchmark U.S. crude for immediate delivery -- rose to $17 a barrel several days ago as U.S. oil companies stepped up purchases to meet the increasing demand for gasoline by vacationers and motorists taking advantage of lower gasoline prices. The price has since slipped about a dollar a barrel.

"I don't think prices are on their way up to $28 per barrel," said Robert Wescott, senior economist for Wharton Econometrics. "We're looking at a couple of years of oil in the $15 to $20 range."

Jones also suggested that some oil producers outside of the Organization of Petroleum Exporting Countries, such as Norway, may agree to help prop up prices soon. And if economic growth in the United States and Europe picks up, it would put further upward pressure on the price of oil, Jones said.

The inflation rate, excluding the effect of oil prices, is still about 3.5 to 4 percent, but oil prices shouldn't contribute significantly to price increases until the end of the decade, Wescott said.

Excluding energy, prices rose 0.4 percent in April, about average for the previous 12 months, Labor said. Energy prices continued to decline last month, dropping 5.8 percent. Gasoline prices fell 11.3 percent last month and 26.5 percent in the past three months.

Gasoline prices are now 34.9 percent below their peak level of March 1981, and are comparable to prices in the middle of 1979, Labor said.

The Labor Department said that new-car prices rose 0.6 percent last month, in part reflecting price increases for imported cars. The price of used cars fell 0.8 percent, the third consecutive monthly decline; they are now 5.6 percent below their level a year ago. Automobile finance charges declined 1.8 percent, while automobile insurance costs rose 0.8 percent last month and were 13.5 percent higher than a year ago.

Food prices rose 0.3 percent in April, but were still down at an annual rate of 0.9 percent for the last three months. Grocery store prices rose 0.2 percent, the first increase since January. Fresh fruit and vegetable prices rose, while beef prices declined for the fourth consecutive month.

Housing and apparel costs each rose 0.3 percent in April. Medical care continued to climb, increasing 0.6 percent last month and at a 9.6 percent annual rate for the past three months.

The consumer price index in April stood at 325.3, which means that goods costing $10 in 1967 cost $32.53 last month. Another CPI measure used in some collective bargaining contracts fell 0.4 percent to 320.4.

In a separate report, the Commerce Department reported that Americans' personal income rose 1.2 percent in April, following a 0.2 percent rise in March, largely because of subsidy payments to farmers. Without the subsidies, personal income increased 0.2 percent.

Increases in personal income generally indicate that consumers will increase their levels of spending in coming months. Personal consumption increased 0.3 percent, following a 0.1 percent rise the previous month.