Producer prices rose 0.6 percent last month, bringing the increase over the last year to 11.7 percent, the Labor Department reported yesterday. Meanwhile unemployment slipped sightly again in December to 7.4 percent of the labor force, the department also said.

But in contrast to this, the Federal Reserve yesterday reported a decline in the nation's money supply for the fifthe successive week, indicating that the economy could be heading downward again. The narrowest measure of the money supply, M1-A, dropped by $2.3 billion in the week ending Dec. 31 to a seasonally adjusted level of $381.4 billion. M1-B, which includes checking accounts at nonbank savings institutions, dropped by $2.5 billion to $406.8 billion, the Fed said.

The latest fall wipes out the jump in the money supply in October and November, bringing the money numbers back below their average level for September.

Many experts now believe that President-elect Ronald Reagan will come into office just as the economy slips back and inflation looks ready to accelerate again. The latest money figures show "the economy is sliding very rapidly and the pressures on the financial markets are gone," Allen Sinai of Data Resources Inc. said yesterday.

Reagan yesterday reiterated his basic beliefs on the economy in an article in the Wall Street Journal. Promising a shift away from public-sector intervention and towards freeing up regulations and taxes to help business and industry, Reagan said "I interpret the 1980 election as a call by the people to draw a bottom line on the recent era of pessimism, no-growth, excessively high taxes and over-regulation."

While cautioning that it would take some time for his policies to take effect, Reagan promised that the "first positive effects [of the tax cuts he has supported] should be felt soon and they will build upon themselves." But "though we shall move deliberately, with clearly identified goals, we won't do so in haste," he went on.

Last year's 11.7 percent annual increase for producer prices of finished goods compared with the 12.9 percent inflation rate in the previous year. Consumer food and energy prices rose more slowly last year than in 1979, but nonfood, nonenergy goods accelerated from the previous year.

The 0.6 percent producer price rise recorded for last month matched that in November. It is equivalent to an annual rate rise of 7.8 percent. Prices of intermediate goods jumped by 1.3 percent in December, the Labor Department said. Nearly half of the rise in this index was caused by a 17 percent leap in motor vehicle parts prices.

Food prices dropped last month at all levels of production, Janet Norwood, commissioner of the Bureau of Labor Statistics, told congressmen yesterday at the regular monthly meeting of the Joint Economic Committee. But the news on energy was not so cheering: Energy prices rose by more than 1 percent in December for the second month in a row, and Norwood warned that "energy prices have once again begun to accelerate."

"The magnitude and duration of changes in energy prices in the coming month will be an important factor in the inflationary climate for 1981," she told the JEC.

Food prices also are expected to rise during the coming months as a result of last year's drought and bad harvest. Part of the reason for their moderation in the last few weeks has been heavy slaughter of farm animals. This will only be temporary.

Meanwhile, Norwood commented that although the unemployment rate in December was little changed from November, "Some improvement in the economy occurred as both the number of payroll jobs and factory work week continued to rise." Total employment as measured by the Labor Department's survey of households was little changed last month, but businesses reported 200,000 more jobs, she said.

Many economists now forecast a rise in unemployment in the early months of this year as growth slows. They believe that the rate may top 8 percent by the middle of the year. Last year unemployment jumped from 6 percent in December 1979 to 7.6 percent in May 1980. Since then, the rate has fluctuated between 7.4 percent and 7.6 percent.

The labor force grew by 900,000 during 1980 to 105.1 million, a pace "substantially below" that of earlier years, Norwood said. Considerably fewer women entered the work force. This could partially reflect the depressed economy.

Meanwhile, the money supply numbers confirm that the Fed probably kept growth of both M1-A and M1-B within its targets for 1980. In the last four weeks the narrower measure grew on average by only 0.9 percent at an annual rate from three months earlier. On the same measure M1-B grew by a 3.1 percent annual rate.