The people who run E.I.L. Instruments Inc. thought the recession would pass them by, but like so many other companies last year, E.I.L. learned otherwise. Sales grew from $22 million to $24 million, but profits dropped to $687,000 from $707,000.

Citing its obligation to stockholders, E.I.L. decided to cut salaries of all senior management, effective Jan. 1, and reduce the number of employes 5 percent, said Pat O'Brien, vice president and chief financial officer.

O'Brien said yesterday the layoffs were from all areas of the company, which had 375 employes and is based in Sparks, Md. E.I.L. manufactures and distributes measurement and testing equipment.

"We even felt like we should have taken these steps a little sooner," O'Brien said. "But we really thought we were recession-proof, that we'd escape."

Cost increases during 1982 made E.I.L. change its mind and decide to trim operations. "The sales growth we'd need this year, we thought was unattainable in a recessionary economy, so we had to cut costs," he said.

"Since we went public April 8, 1981, we've felt like we have an obligation to shareholders to give them a respectable yield," O'Brien said, estimating stockholders' return on equity as 10 or 11 percent for 1982.

Fourth-quarter profits slipped to $82,000 (6 cents a share), compared with $257,000 (18 cents) in 1981. Revenues increased slightly to $6.14 million from $6.11 million. Per-share earnings for all of 1982 were 48 cents, down from 53 cents for 1981.

In addition to its cost-cutting measures, E.I.L. has taken on some product lines that it thinks will be less price-sensitive.

C3 Inc., a microcomputer manufacturer in Reston, reported record third-quarter revenues yesterday of $17.1 million, 29.4 percent higher than the $13.2 million the year before.

Third-quarter profits dropped 3 percent to $2.1 million (25 cents a share) from $2.2 million (26 cents) in 1981. President John G. Ballenger attributed the earnings decline to legal fees incurred in the Army's continuing investigation of C3 and to start-up costs for two divisions. One division is Micro Products Co., a computer terminal manufacturer in Sterling; the other is Tempest Technologies Inc., a producer of secure computer systems.

For the nine months ended Dec. 31, profits declined to $5.3 million (63 cents) from $5.9 million (70 cents) the year before. Revenues in that period increased to a record $39.8 million, 13.2 percent higher than the $35.1 million last year.

First Virginia Banks Inc. posted record earnings for the year and the fourth quarter, company officials said yesterday.

Net income was $25 million ($2), a 34 percent increase over 1981's earnings of $18.7 million ($1.49). Before securities transactions, First Virginia's income was $25 million, compared with $19.7 million last year.

For the last quarter, First Virginia earned $6.6 million (53 cents), compared with $4.1 million (32 cents) in 1981. Before securities transactions, the company earned $6.6 million, compared with $4.5 million last year. All 1981 figures have been restated to account for two banks acquired in 1982.

Total assets for the Falls Church-based company reached $2 billion at year's end, up 12 percent from 1981. Deposits grew to $1.8 billion and loans increased to $1.2 billion.

Dominion National Bank Corp., with headquarters in Roanoke, reported a decline in earnings for 1982, dropping to $17.7 million ($1.79) from $22.5 million ($2.29) in 1981, a difference of 21.6 percent. Before securities transactions, Dominion earned $22.5 million, compared with $22.9 million last year.

Fourth-quarter net income dropped to $2.95 million (30 cents) from $5.7 million (58 cents).

Before securities transactions, Dominion had income of $5.1 million in the fourth quarter, down 17 1/2 percent from $6.2 million in 1981.

Assets grew 13.2 percent to $3.2 billion from $2.8 billion. Deposits at year-end were $2.5 billion and loans were $1.7 billion.