Peoples Drug Stores posted a 16 percent gain in profits yesterday for the first quarter of its fiscal year, and USAir reported a January loss less than half as big as its deficit a year ago.

Peoples' profits for the 16 weeks ended Jan. 17 totaled $4.069 million ($1.06 a share), up from $3.51 million (95 cents) in the same period a year ago. Sales climbed 22 percent to $207.2 million from $169.4 million.

The profit increase came in spite of higher interest rates and a substantial increase in the charge for last-in, first-out accounting, said Peoples' Chairman Adrian C. Israel.

Unlike most retailers, who end their fiscal year with the Christmas season, Peoples starts its year with the most profitable period of the annual cycle, then follows it with three 12-week quarters. As a result, the first quarter produces a much bigger than usual share of annual profits and sales.

At USAir, January revenues totaled $80 million, up 24 percent from last year's $64.5 million, and the loss for the month fell to $870,000 from $2.125 million.

American Realty Trust of Arlington reported its earnings for the quarter ended Dec. 31 climbed to $77,348 (3 cents a share) from $61,252 (3 cents) a year earlier.

Total revenues for the quarter amounted to $1.7 million. The operating profit included a $25,000 gain from sale of property and a tax credit for previous losses of $37,000. Negotiations are under way to sell additional property and refinance the trust's debts, a spokesman said.