Delta Air Lines said Thursday it reversed a loss from a year ago and Southwest Airlines said its profits in the January-March quarter rose as both carriers' costs rose only slightly despite higher fuel prices.

Delta Air's profit came against a loss last year generated by recognizing a lower value for its fleet of L-1011 jets. Not counting one-time expenses or gains, Delta's earnings would have been more than triple a slim profit last year.

Delta Air Lines Inc.

Delta earned $187 million, or $2.52 per share, on revenues of $3.42 billion. A year earlier the airline lost $298 million, or $5.77 per share, on revenues of $2.96 billion.

Despite the good results, Delta's stock dropped $5 to close at $93.12 1/2. Airline stocks have risen sharply in recent weeks and analysts said Thursday that profit taking was sending some of the bigger carriers lower. Stock in United Airlines parent UAL was also off 3.5 percent.

Delta lost money last year largely because of a one-time $556 million charge to cover the depreciated value of its L-1011 fleet.

Delta's traffic numbers were higher in the quarter and it flew with fewer empty seats. Its cost of flying one seat one mile rose 1.4 percent with a 15 percent increase in fuel prices.

For the first three quarters of Delta's fiscal year, the airline reported a profit of $552 million, or $7.30 per share, on revenues of $10.05 billion. A year earlier the company lost $72 million, or $1.39 per share, on revenue of $9.1 billion.

Southwest Airlines Co.

Southwest profit for the quarter ended March 31 was $50.9 million, or 34 cents a share, compared to $33 million, or 22 cents a share, in the same period last year. Revenues were $887 million compared to $773 million in the first quarter of 1996.

Southwest averaged 62.1 percent of its seats full in the quarter, up from 60.5 a year earlier. The airline said it doesn't expect to match the 67 percent load factor reached in the April-June quarter a year ago.

``Additionally, the March reimposition of the federal ticket tax may adversely impact revenue growth during second quarter 1997. The ticket tax was not in effect during second quarter 1996,'' said chairman and chief executive Herb Kelleher.

Southwest's stock rose 87 1/2 cents to $26.12 1/2 on the New York Stock Exchange.