The nation's money supply, an important economic indicator, dipped last week after sharp increases the two previous weeks.

According to the Federal Reserve, the broad measure of cash in circulation, checking and NOW account balances known as M1-B declined by $100 million in the week ending April 15. For the latest four weeks, M1-B averaged $425.6 billion on a seasonally adjusted basis, an 11.8 percent rate of increase in the statistical quarter. The previous week's rise was revised upward by $400 million to $5.2 billion.

Analysts caution against trying to interpret one week's figures. Some theorize, however, that bank deposits were returning to normal after spurting up for two weeks as individuals added money to their accounts in antipation of paying income taxes.

For the same period, the narrower gauge called M1-A declined by $2.2 billion to a seasonally adjusted average of $363.4 billion. The Fed revised the figure for the week ending April 8 upward by $300 million.

In other economic indicators last week:

Commercial and industrial loans on the books of the nation's largest banks fell $68 million, compared with a drop of $1.2 billion the previous week.

Borrowings from the Federal Reserve System were also down, to $715 million from $993 million.

Finally, the Federal Home Loan Bank Board is expected to announce on Monday a very large outflow of funds during March from financially troubled savings and loan associations.