Garfinckel, Brooks Brothers, Miller & Rhoads, Inc., said its best second-quarter sales ever produced profits nearly four times those of the previous year.

Sales for the period ended July 30 increased form $64.7 million to $68 million: earnings jumped from $111,000 (2 cents a share) to $406,000 (8 cents).

For the first six months, sales were $131.9 million, up 4.9 per cent and net income was $1.187 million (26 cents) against $978,000 (22 cents) for the same period last year.

Sales were boosted by a new Brooks Brothers store in Seattle, which opened June 9, but not yet include operations of Ann Taylor, a chain of women's specialty apparel shops which the company has agreed to acquire.

The quarterly earnings increase was against a weak quarter last year, noted David R. Waters, president. "Based upon positive second-quarter and current salas trends and the fact that our six-months earnings are ahead of last year, we are somewhat encouraged about sales and earnings for the fall," he said.

Potomac Electric Power Co. earned $39.6 million during the first seven months for the corresponsing period in 1976. Because of an increase in the number of shares outstading, earnings per share declined from 85 to 81 cents.

Gross revenues for the utility company's first seven months were $358 million, against $304 million last year.

For the past 12 consecutive months, Pepco earned $77.1 million on revenues of $603 million compared with $69.2 million on revenues of $528 million.