NEW YORK, SEPT. 25 -- The stock market, faced with persistent uncertainty about the interest-rate outlook, turned in a mixed showing today in the slowest trading in three weeks.

The Dow Jones average of 30 industrials edged up 3.75 to 2570.17, finishing the week with a net gain of 45.53 points.

Volume on the New York Stock Exchange came to 137.95 million shares, down from 162.15 million Thursday and the smallest total since 129.07 million changed hands Sept. 4.

The slowdown in activity over the past two sessions was attributed in part to the observance of Rosh Hashana, the Jewish New Year.

After rising sharply on Tuesday and Wednesday, stock prices pulled back a bit Thursday as interest rates came under new upward pressure in the credit markets. Rates were mixed in today's activity.

Analysts said many traders were proceeding cautiously as they awaited meetings this weekend in Washington among financial policymakers of the United States and its major trading partners.

Irving Bank Corp. jumped 25 3/4 to 78. Bank of New York offered to acquire Irving for a package of cash and stock.

Takeover speculation prompted buying in several other issues, including Hershey Foods, up 3 at 35 1/4, and IC Industries, up 3 7/8 at 38.

Auto stocks were a notable weak spot. Ford Motor dropped 3 3/4 to 97 1/4; Chrysler 1 7/8 to 41 1/8, and General Motors 1/8 to 84 1/2. Analysts have described recent sales trends in the industry as disappointing.

Some regional telephone issues, by contrast, gained ground. Pacific Telesis rose 1 1/8 to 31 3/4; Southwestern Bell 3/4 to 43 3/4, and U.S. West 7/8 to 58 3/4. Southwestern Bell and U.S. West were among 43 NYSE issues hitting new 52-week highs, against 66 new lows. Homestake Mining climbed 7/8 to 47 5/8. The company's directors approved a 2-for-1 stock split and a dividend increase.

Declining issues outnumbered advances 8 to 7 in the overall tally on the NYSE. The exchange's composite index added 0.28 to 179.14.

Nationwide turnover in NYSE-listed issues, including trades in those stocks on regional exchanges and in the over-the-counter market, totaled 163.84 million shares