The stock market staged a mild rally today as traders anticipated some favorable news on the money supply.

The Dow Jones average of 30 industrials, up more than 5 points at its mid-day peak, wound up with a 3.81 gain at 843.64.

New York Stock Exchange volume reached 36.49 million shares, up from 33.79 million Wednesday.

Some buying interest was attributed to expectations that the Federal Reserve would report a drop in the basic measure of the money supply, offsetting some of the previous week's record $6.9 billion increase.

As it turned out, the statistics issued shortly after the close showed a $2.6 billion decline. The growth rate of the money supply is watched closely by Wall Streeters because it is a primary factor considered by the Fed in setting monetary policy.

The Amex market value index was up 2.26 at 199.93 for the day, after briefly crossing the 200 level for the first time at mid-session. The index was established at 100 in 1973.

The daily tally on the NYSE showed advances outstripping declines by about an 8.5 margin, and the exchange's composite index rose .26 to 57.91.

Nationwide turnover in NYSE-listed issues, including trades in those stocks on regional exchanges and in the over-the-counter market, came to 40.92 million shares.

Standard & Poor's index of 400 industrials was up .57 at 113.30, and S&P's 500-stock composite index added .46 to 102.09.

The NASDAQ composite index of over-the-counter issues rose .26 to 136.71. CAPTION: Graph, The Market, June 21, The Washington Post