The stock market closed its busiest week in history with a whimper today. The Dow Jones Industrial Average fell by 3.77 points, and volume dropped to 80.2 million shares, below the 100-million-share mark for the first time in six sessions.

The declines brought the closely watched Dow industrial average to 993.10, up 6.25 points for the week, after two days of losses of almost 22 points. Volume for the five days totaled 595.07 million shares, shattering the previous one-week mark of 549.8 million shares set in August.

Nationwide turnover in NYSE-listed issues, including trades in those stocks on regional exchanges and in the over-the-counter market, totaled 94.48 million shares.

The New York Stock Exchange composite index fell 0.49 point to 76.74, while the average price per share was down by 20 cents. Some 685 stocks advanced, and 951 declined.

Standard & Poor's index of 400 industrials fell 1.09 points to 149.27, and S&P's 500-stock composite index was off 1.00 point at 133.57.

Market analysts attributed the pullbacks in stock prices primarily to profit taking, although it was clear that many traders were taking a breather after a hectic four days.

Indications that Federal Reserve Board policy would permit a steady lowering of interest rates pumped the market up late last week, and those gains continued through yesterday, when the market pulled back from an 18-month high in the Dow Jones Industrial Average of 1,015. There was little indication on Wall Street today that the two-day decline was anything but profit-taking after the dramatic runups on blue-chip stocks.

"You can't deny the trends that are in place," noted Stewart Pillette, associate research director at Drexel Burnham Lambert Inc. "We are just seeing some resting here, but the overall trend in rates is still down, so the overall trend in multiples or returns is still up."

The market's retrenchment over the past two trading sessions followed two sharp upturns over the past two months. The first, in August, pumped the Dow industrials from a 1982 low of 776.92 on Aug. 12 into the low 900s, where that market indicator stayed until last week, when the Dow surged 79 points to 986.85.

This week the industrial average moved over the 1,000 mark on Monday, giving it a gain of 110 points in just four days. There was some exuberent talk on Wall Street of the indicator breaking its nearly-10-year-old, all-time record of 1,051 but, as has happened repeatedly in recent years, traders became edgy by midweek, opting instead to take sizable profits on the blue-chip, widely traded issues that had dominated the rally.

As the week closed today, the most widely held view among analysts seemed to be that, for now, at least, the Dow industrial average had peaked, but that a second October spurt near the end of the month--assuming continued downward interest rate movement continues--might bring that indicator toward its all-time high.

Meanwhile, in the bond markets, which have been just as turbulent as the stock markets over the past week, prices fell lower through the day's trading, although government offerings picked up as much 3/8 point after the Federal Reserve released figures showing that the money supply rose by $6.9 billion, a figure within most traders' expectations.

The action also cooled today on the American Stock Exchange, where volume dropped from 10.3 million shares yesterday to 6.8 million today. The Amex index was off 0.51 point, closing at 313.88, with the average price per share down two cents. There were 308 issues advancing, and 309 declining.

On the over-the-counter market, the NASDAQ composite index was off 0.39 point, closing at 204.46. Declining issues there outnumbered advancing ones by 590 to 565, with 2,177 over-the-counter securities unchanged.