Planning Research Corp. said yesterday its earnings rose 27 percent in its fiscal second quarter despite a 3 percent decline in revenues. A spokesman said the drop in revenues was primarily the result of the sagging economy.

The McLean-based company attributed the rise in profits to better results in two of its four operating divisions and the absence of one-time losses that affected last year's figures.

PRC, which provides a variety of professional engineering, computer and management services to the government and corporations, said it earned $2.2 million (32 cents a share) in the quarter ended Dec. 31, up from $1.8 million (26 cents) a year earlier. Revenues in the period, however, slipped to $77.5 million from $79.8 million.

In the six months ended Dec. 31, PRC earned $4.3 million (63 cents), a 19 percent rise from $3.6 million (53 cents) a year earlier. Revenues in the half dipped 2 percent to $155.5 million from $158.9 million.

First American Bank of Maryland posted record earnings for the fourth quarter and for 1982. Net income for the year was $2.7 million ($2.70), a 78.8 percent gain over the $1.5 million ($1.51) earned in 1981.

Before securities transactions, the bank had 1982 earnings of $2.8 million, compared with $1.5 million.

Fourth-quarter net income increased 332.7 percent to $792,000 (80 cents) from $183,000 (19 cents) in 1981. Before securities transactions, the company had earnings of $808,000, compared with $183,000 in the final quarter of 1981.

Pargas Inc. posted earnings of $12.4 million ($2.97), down from $13.1 million ($3.15) during 1981. Revenues slipped to $271.2 billion from $281.2 million.

Fourth-quarter profits were $4.4 million ($1.07), compared with $5.4 million ($1.30).

N.L. Langley, president and chief executive of the Waldorf, Md., company, said LP gas operations had record earnings last year, but coal operations posted a loss.

Bank of Maryland had 1982 net income of $1.3 million ($4.49), 51.6 percent higher than the earnings of $835,890 ($3.08) in 1981.

Fourth-quarter net income jumped from $219,562 (81 cents) in 1981 to $329,947 ($1.17). Year-end assets were $105.5 million, and deposits were $92.4 million