The stock market rebounded from some early selling to finish mixed today, thanks to some late buying of blue chips.

The Dow Jones average of 30 industrials, down about 13 points at midday, closed with a 3.16-point gain at 1,868.94.

Volume on the New York Stock Exchange slowed to 117.04 million shares from 123.09 million Tuesday.

The Dow Jones industrials dropped 8.41 points in the week's first two sessions. Analysts said enthusiasm for stocks slackened lately as evidence mounted that the pace of business activity remains subdued.

Before the opening today, the government revised downward its estimate of economic growth in the first quarter of this year.

The Commerce Department said the gross national product grew at an annual rate of 2.9 percent, after adjustment for inflation and seasonal factors, in the January-March period, rather than the 3.7 percent rate estimated earlier.

Though the news came as no surprise on Wall Street, analysts said it was one more indication that growth for 1986 as a whole is lagging behind expectations, including the projections of President Reagan's administration.

Data on personal income and consumer prices due out this week are expected to provide more evidence that the economy is weak.

The market's strength late in the session was seen as encouraging by some analysts, who noted that the bears had been in control for the first half of the week without making much headway.

But observers still had some questions about whether significant buying interest would be seen before the close on Friday, which marks the end of trading in a set of stock-index futures and options that are expiring.

This so-called "witching hour" often has produced wide swings in the market in the past as professionals at brokerage houses complete program trades involving both the options or futures and individual stocks.

Computer and technology stocks slumped early in the session in selling attributed to the latest signs of weakness in the economy. Some of them rallied, however, to lead the late recovery by the market.

Texas Instruments dropped 3 to 119 1/2, but Digital Equipment rose 1 7/8 to 83 7/8 and Hewlett-Packard rose 1/8 to 41 7/8.

Traders said late buying in the technology sector was prompted by talk that International Business Machines Corp. officials meeting with securities analysts in California would not be as negative about their earnings as some analysts have anticipated. IBM, which was down more than 2 points for much of the day, closed off 1/2 at 146 3/8.

Upjohn also rallied near the close, finishing unchanged at 96 3/4 after trading as low as 93 1/4. The stock fell 4 7/8 Tuesday, when news surfaced of a letter from the Food and Drug Administration to Upjohn contending that a company press release exaggerated the effectiveness of its drug minoxidil as a treatment for male baldness.

The semiconductor group remained under pressure after Morgan Stanley cut its earnings estimates for the companies. Motorola fell 1 to 38 1/2, Advanced Micro Devices lost 1/4 to 21 1/2 and Texas Instruments dropped 3 to 119 1/2.

Declining issues outnumbered advances by about 5 to 4 on the Big Board. The exchange's composite index gained 0.26 to 140.63.

Nationwide turnover in NYSE-listed issues, including trades in those stocks on regional exchanges and in the over-the-counter market, totaled 140.15 million shares.

Standard & Poor's index of 400 industrials rose 0.53 to 274.13, and S&P's 500-stock composite index was up 0.64 at 244.99.

The Nasdaq composite index for the over-the-counter market fell 1.04 to 395.24.

Prices were mixed in active trading on the American Stock Exchange.

Horn & Hardart led the Amex actives, rising 1 3/4 to 14 1/4. Consolidated Stores, followed, jumping 1 3/4 to 21 3/8. Dome Petroleum was third, unchanged at 1.

The American Stock Exchange index rose 0.11 to 280.61. The price of an average share was unchanged. Declines outnumbered advances by 336 to 228 among the 809 issues traded. Composite volume totaled 13.5 million shares, compared with 17 million shares traded Friday.