Dominion Resources Inc., the holding company for Virginia Power Inc., yesterday reported a 66 percent gain in second-quarter profit.

The performance of Virginia Power, an electric utility with 1.7 million customers in Virginia and North Carolina, was the reason for the sharp increase in profits, Dominion said. Its operating expenses fell as the utility completed a cost-reduction program and major nuclear power plant maintenance activities. In addition, higher rates took effect May 1.

For the second quarter, Dominion Resources reported a profit of $85.7 million (85 cents a share), compared with $51.5 million (52 cents) in the 1989 second quarter. Revenue fell 0.5 percent, to $846.3 million from $850.8 million. In the first half, the company earned $212.7 million ($2.11), up 47 percent from $144.6 million ($1.47) in the year-earlier first half. Revenue fell 3.2 percent, to $1.72 billion from $1.77 billion.

LaFarge Corp., a Reston-based cement and construction-material producer, said its earnings dropped 23 percent in the second quarter, largely because of slowdowns in the construction market in Canada, where the company has many operations. LaFarge's profit dropped to $29.4 million (57 cents) in the quarter from $38 million (76 cents) a year ago. Revenue, however, was up 9 percent, to $429.8 million from $394.5 million a year ago. In the first half, LaFarge lost $11 million, compared with a profit of $18.3 million (37 cents) a year earlier. Revenue rose 11 percent, to $669.2 million from $601.9 million a year ago.

Danaher Corp., the Washington-based holding company for a variety of industrial operations, said profit dropped 9 percent in the second quarter, to $11.5 million (46 cents) from $12.5 million (53 cents) a year ago. The company attributed the drop to weakness in transportation markets, to which it supplies brakes and other parts, as well as to the effects of changes in the Brazilian economy brought on by wage and price controls, devaluation of the cruzeiro and other measures. Although only 1 percent of Danaher's revenue comes from Brazil, the company said most of the drop in earnings was the result of write-offs in its Brazilian operations. Revenue in the quarter rose 2 percent, to $203 million from $198.6 million. In the first half, Danaher earned $22.2 million (92 cents), off 8 percent from $24 million ($1) a year ago. Revenue in the half slipped 3 percent, to $383.6 million from $393.6 million.