Texaco Inc., the nation's third-largest oil company, said yesterday its first-quarter profit dropped 13.1 percent from a year ago while sales fell 21.7 percent.

Despite a big improvement in earnings from its refining and marketing operations, Texaco's overall profit slipped to $319 million, or $1.24 a share, from $367 million, or $1.41 a share, reported in the comparable quarter last year.

Revenue declined to $10.2 billion from $13.0 billion.

Texaco's report fit a pattern of generally declining first-quarter profits in the oil industry, although it had a bigger improvement in refining and marketing than many other companies. Texaco said it earned $36 million from those operations compared with a loss of $145 million in the same three months last year.

Earnings from petroleum and natural gas production and exploration slipped to $371 million from $424 million.

Aetna Life & Casualty Co. earned $1.58 a share in the first quarter, up from $1.08 a year earlier.

The big gain resulted from realized capital gains of $43.1 million in contrast with capital losses of $13.2 million a year.

Net income rose to $162.1 million from $87.4 million.

Aetna also had a gain in operating earnings to $119 million from $1.01 million but operating earnings per share fell to $1.15 from $1.25 because of substantially larger number of shares outstanding this year.