The nation's money supply fell by $1.9 billion last week to a seasonally adjusted $452.5 billion, after a record rise in the previous week, the Federal Reserve reported yesterday.

The Fed revised the previous week's jump in the money supply up by $800 million, to $7.9 billion. The new data "show clearly there has been no meaningful washout of the early April bulge," William Sullivan of the Bank of New York said yesterday. This could keep interest rates from easing next week, analysts said.

In a separate report, the Treasury announced that federal outlays exceeded receipts by $18.255 billion in March, bringing the deficit in the first half of this fiscal year to $71.9 billion.

The new figures go some way toward wiping out an apparent improvement in the deficit picture cited by some tax-cutting advocates in defense of President Reagan's tax policies.

Based on an extrapolation of the first five months of the fiscal year, the Heritage Foundation said earlier this month that the deficit for fiscal 1982 may be only $65 billion to $75 billion rather than the $100.5 billion predicted by both the Treasury and the Office of Management and Budget.

The foundation noted that, between October and February, revenues were running 13 percent above the same period a year earlier, while outlays were up by only 10 percent. However, yesterday's figures have changed that calculation to an 11.1 percent increase in receipts and an 11 percent increase in outlays.

"These figures do not change our view" about the likely deficit, OMB spokesman Edwin Dale said yesterday. Administration officials, including Treasury Secretary Donald Regan, have rejected the suggestion that their deficit projection is too high. Private analysts agree that the 1982 budget gap probably will top $100 billion.

The Treasury's cash position nevertheless may be good enough to delay the need for a new increase in the debt ceiling until July, financial market sources said yesterday. The debt ceiling bill is seen as a deadline for Congress to act on the 1983 budget.

The Fed report also showed that M1, which includes cash in circulation and checking accounts at all financial institutions, was up by an annual rate of 4.3 percent in the latest four weeks from a comparable period three months earlier. The Fed has refused to comment on the big bulge in the money supply this month, which was larger than many dealers expected.

However, the Fed doesn't believe that the seasonally adjusted weekly money supply figures give a reasonable guide to the underlying trend in money growth and plans to stop publishing these figures soon.