Producer prices rose by 0.9 percent in January, the biggest monthly rise since last August, the Labor Department reported yesterday. The increase came despite stable food prices for the month.

The January rise, which represents a 10.8 percent increase at an annual rate, does not reflect the latest round of oil price increases resulting from President Reagan's decision to decontrol oil prices last month. Decontrol probably will add about 10 percent to fuel prices, experts believe.

In December producer prices rose 0.5 percent. Much of the change between the two months came because food prices stopped falling in January. Energy prices also accelerated during January, rising by 2.7 percent, as wholesale gasoline and home heating oil prices "increased by more than twice as much as in December," Labor Department analyst Craig Howell said. t

"The one moderating influence we have is food prices," he went on. Analysts have been expecting food prices to rise because of the drought last year, but so far their effect at the wholesale level is still to hold prices down. Producer foodstuffs prices dropped by 1.1 percent last month compared with a fall of 2.6 percent in December.Consumer food prices held steady.

Meanwhile the Federal Reserve announced a further drop in the narrow measure of the money supply in the week ending Feb. 4. M1-A, which includes cash and notes in circulation and demand deposits at banks, fell by a seasonally adjusted $500 million, the Fed said. The M1-B measure rose by $1.8 billion in the week to a reasonally adjusted $415.1 billion level.

The introduction of nationwide negotiable order of withdrawal (NOW) accounts, which pay interest on checking accounts, has made the latest figures hard to interpret. M1-B includes NOW accounts as well as all checking accounts at banks and thrift institutions.

However, its average level for the past four weeks was up at only a 2.2 percent annual rate from three months earlier the Fed report showed. M1-A dropped at an annual rate of 18.1 percent over the same period, presumably as people switched their money out of nonearning demand deposits and into interest-bearing NOW accounts.

The figures suggest that the rapid runup in interest rates last month as the Fed tightened credit has helped to cut back on the money supply. The present slow decline in rates should continue as loan demand eases and the markets see the money figures decline.

The Fed is due to announce new targets this month for 1981 money growth. These are expected to be somewhat lower than the 1980 targets.

Economists are expecting only sluggish growth this year, with perhaps a drop in output in the next few months, followed by a revival towards the end of the year. But no dramatic improvement in inflation is forecast. The producer prices which are most sensitive to demand are those of crude materials, which dropped by 1 percent in January.