Potomac Electric Power Co. raised its dividend yesterday by 5 percent and reported its revenues last year passed $1 billion for the first time.

Pepco's earnings increased to $109.7 million ($2.14 a share) for 1981, up from $104.1 million ($2.10) in 1980, while annual revenues climbed to $1.0005 billion from $856 million the year before.

The power company announced its quarterly common stock dividend will be boosted from 40 to 42 cents a share effective with the dividend to be paid March 31 to shareholders as of Feb. 25. Pepco's board also approved the regular quarterly dividend on preferred and preference stocks to be paid March 1 to shareholders as of Feb. 3.

For the fourth quarter of 1980, Pepco earned $23.3 million (43 cents) on revenues of $225 million, compared with profits of $20.6 million (39 cents) on revenues of $208 million in 1980.

First Virginia Banks posted an 18 percent gain in net earnings for 1981 as profits of its 178 banking offices climbed to $17.9 million ($1.60) from $15.1 million ($1.42) the prior year.

While net earnings grew 18 percent, income before securities transactions jumped 23 percent to $18.9 million ($1.70) from $15.4 million ($1.45).

For the fourth quarter, net income fell to $3.9 million (35 cents) from $4.6 million (43 cents), while income before securities transactions slipped to $4.3 million (39 cents) from $4.7 million (44 cents).

Farmers and Mechanics National Bank of Frederick reported profits for 1981 of $2.14 million ($3.43) compared with $2.05 million ($3.29) in 1980. Loans rose to $155.3 million from $145.83 million, and deposits increased from $215.61 million to $229.03 million.

Mercantile Bankshares Corp. reported 1981 profits of $20.01 million ($3.57 a share), up from $16.14 million ($2.86) in 1980. At the end of 1981, Mercantile had assets of $1.5 billion, compared with $1.3 billion a year earlier. Deposits were $1.3 billion on Dec. 31, up from $1.07 billion.

Computer Data Systems Inc., a professional services and data processing company based in Bethesda, said net income for the six months ended Dec. 31 rose to $552,500 (76 cents a share) from $525,500 (72 cents) a year earlier, as revenues increased 29 percent to $10.8 million.

Wapora Inc., an environmental, energy and economic studies firm in Chevy Chase, reported a net loss of $7,600 in the six months ended Dec. 27 compared with profits a year earlier of $135,082 (23 cents a share). Revenues declined to $3 million from $3.8 million.