The stock market settled for a mixed showing today as the slow-paced rally that began in Tuesday's session quietly coasted to a stop.

The Dow Jones average of 30 industrials closed unchanged at 834.55 after a 7.54-point rise on Tuesday.

Losers nosed out gainers by about an 8-to-7 margin in the daily tally on the New York Stock Exchange.

At the American Stock Exchange, a string of eight consecutive advances was broken as the market value index eased 0.01 point to 163.34.

Except for the recent mild upturn on the Amex, the market has been sluggish and trendless for most of February in the face of the upheaval in Iran, border fighting between China and Vietnam, and continued uncertainty about such economic problems as inflation.

Among the day's volume leaders, Diamond Shamrock rose 1/8 to 21 1/8. The company said proxies received to date suggest that its holders will approve the proposed acquisition of Falcon Seaboard at a special meeting Thursday.

UV Industries gained 1 to 33 7/8 in active trading. The company has set a meeting March 26 for a vote by shareowners on a plan to liquidate the company and distribute the proceeds to stockholders.

American Telephone slipped 1/4 to 63 3/4 after having climbed steadily in recent sessions in anticipation of a dividend increase. When the company announced a boost in the quarterly payout today from $1.15 to $1.25 a share, some traders apparently cashed in profits, following the longstanding Wall Street precept, "Buy on the rumor, sell on the news."

Big Board volume totaled 26.05 million shares, up from 22.01 million Tuesday, and the exchange's composite index slipped 0.16 point to 55.58.

Nationwide turnover in NYSE-listed issues, including trades in those stocks on regional exchanges and in the over-the-counter market, came to 29.65 million shares.

Standard & Poor's index of 400 industrials dropped 0.39 point to 110.51, and S&P's 500-stock composite index was off 0.35 point at 99.07.

Rosario Resources gained 1 1/4 to 24 1/2. The company reported on an independent study of the Canadian oil and gas reserves of its Alamo Petroleum Ltd. subsidiary.

In the over-the-counter market, the NASDAQ composite index eased 0.07 point to 125.76.