The nation's money supply soared by $4.8 billion on the M1-B measure in the week ended April 8, the Federal Reserve reported yesterday. This measure includes cash and checking accounts at all banks and other financial institutions.

The rise followed an increase of $3.4 billion in M1-B in the previous week. However a Fed spokesman yesterday said that even "more than the usual caution" should be applied to considering the figures.

Figures are often volatile at this time of year, he said, and there has been a sharp rise in the volume of negotiable order of withdrawal (NOW) accounts, which probably has distorted the M1-B figures. The narrower measure of the money supply, M1-A, rose by only $200 million.

The administration is forecasting a big fall in interest rates between now and the end of the year, as the economy flattens out. But a week ago two major banks raised their prime rates by half a percentage point to 17 1/2 percent, and mortgage rates recently have begun to climb. The latest rise in the money supply could push rates up when the markets open on Monday.

The M1-B measure averaged growth of 10.6 percent in the latest statistical quarter, the Fed said. But business loans from the nation's large banks fell in the latest week by $1.248 billion. This could be an indicator of recession.

Meanwhile, the Commerce Department reported that personal incomes rose by 0.8 percent in March, probably barely enough to keep pace with inflation.

More money was put into savings during the month, the department said.

Personal income climbed by $17.7 billion in the month, equivalent to a 9 1/2 percent annual rate rise. The increase in February was 0.7 percent, or $16.3 billion.

Despite the slow growth in income, when compared with price rises, the economy kept growing during the first quarter of the year, early reports from Commerce have said. Figures for gross national product, which measures the total output of goods and services in the economy, will be published Monday.

These may not be as buoyant as first expected, since the Commerce Department now has revised downward its estimates of consumer spending in February.

Yesterday's release showed a spending rise of 0.5 percent in February rather than the 1.3 percent originally reported. After adjustment for inflation, spending in February did not change at all, the department said.

William Cox, chief economist at Commerce, said much of the revision was because of lower purchases of gasoline and oil than first were estimated. Many experts believe the economy now is slowing down and may not grow at all in the second quarter. One of the chief elements in growth is consumer spending, and if incomes do not increase rapidly then spending is not likely to be high.

The February revision left savings higher than the department originally believed, at 4.7 percent in both January and February. The savings rate probably was little changed in March, as spending increased by the same proportion as income, the report showed. Savings remain low by historical standards, Cox said.

Administration officials say they hope that a sharp rise in saving will result from its economic program and that the added saving then could be used to increase investment. Saving has fallen in recent months, although not as much as was first thought.

After-tax, or disposable, income rose by 0.7 percent last month, to a seasonally adjusted annual rate of $1.96 trillion, the department reported. This followed a similar rise in February.

Personal spending on durable and nondurable goods slipped in March, the report showed, while spending on services increased.

Cox said it would not be surprising if spending grew much more slowly in coming months than it did at the end of last year.