Potomac Electric Power Co. yesterday reported an increase in first-quarter profits to $18.64 million (34 cents a share) from $17.15 million (32 cents) for the 1980 first quarter.

Revenues rose sharply to $221.66 million from $171.61 million.

Among other reports, National Bank of Washington yesterday reported that first-quarter profits fell to $751,581 (55 cents a share) from $1.36 million ($1.00) for the same quarter last year.

But NBW noted that profits rose slightly over the $750,787 reported for the fourth quarter of 1980.

CACI, an Arlington research firm, reported a 48.3 percent rise in revenues for the firm's third quarter, but profits dropped from $531,549 for the same quarter last year to $471,884.

Avemco Corp., a Bethesda company that is the nation's largest aircraft insurer, reported first-quarter profits of $1.14 million (46 cents a share) compared with $1.06 million (43 cents), the previous record, in the same period last year.

Avemco's principal subsidiary, Avemco Insurance Co., reported a 20 percent increase in premium volume in the quarter to $8.5 million from $7.1 million.

Speaking to analysts in Los Angeles Wwednesday, Avemco Chairman Arnold Johnson noted that profits from underwriting insurance had contributed strongly to the first-quarter gains. He also said insurance lines will be expanded to bolster future growth. One new business would be auto and homeowners insurance for members of the Aircraft Owners and Pilots Assocation, Johnson stated.