Conoco Inc., the ninth-largest U.S. refiner, reported yesterday that its first-quarter profits declined 23.6 percent, primarily because of weak oil demand and high petroleum stocks. The company said earnings dropped to $250.9 million ($2.33 a share), from $328.6 million ($3.05 a share) in the January-March period last year.

Revenues were up 25 percent, however, to $5.4 billion from $4.3 billion.

Ralph E. Bailey, Conoco chairman, said lagging demand for gasoline and other petroleum products coupled with high inventories had prevented Conoco from recovering higher crude costs from the consumer. But increated crude oil production from the North Sea partially offset the negative factors in Conoco's refining and marketing operations, he said.

Conoco's earnings slump followed lower first-quarter results posted by other large U.S. oil companies in the face of the global oil surplus.

Marathon Oil Co., the 16th-largest refiner, announced that its first-quarter earnings plummeted by 50 percent to $69.8 million ($1.17), from $1.39.1 million ($2.30) a year earlier. Revenues rose 6.6 percent to $2.42 billion from $2.27 billion.

Aetna Life & Casualty Co. blamed intense price competition in property-casualty insurance operations for a decline in first-quarter profits to $96.5 million ($1.20) from $122.7 million ($1.52) a year ago.

Aetna announced a month ago that it was raising premium prices on its property-casualty lines. Chairman John H. Filer said individual life insurance earnings in the quarter were level with a year ago, group life earnings up and reinsurance profit 13.1 percent higher, while property-casualty earnings fell 89 percent.

Total premium income was off 16 percent to $2.4 billion. Investment income was up 21 percent to $747 million.