Potomac Electric Power Co. said its earnings dropped almost 9 percent in 1987, with most of the decrease attributable to a one-time gain in 1986 because of the sale of the utility's Virginia operations.

Washington-based Pepco said it earned $208.2 million ($2.11 a share) last year, down from $227.7 million ($2.29) in 1986, when the figures included a $21.7 million extraordinary gain from the sale of the Virginia service territory to Virginia Power. Revenue during the year slipped to $1.33 billion from $1.37 billion a year ago.

For the fourth quarter, Pepco reported a profit of $31.6 million (31 cents), down 17 percent from $38.2 million (38 cents) a year earlier. Revenue in the quarter fell to $273.9 million from $295.5 million.

Lafarge Corp., a cement and construction materials company that moved to Reston from Dallas last year, said its profit nearly tripled in 1987 to $75 million ($1.54) from $25.9 million (58 cents) a year ago. Revenue rose 27 percent in 1997 to $1.2 billion from $963.8 million.

In the fourth quarter, the company's profits more than tripled, to $18.8 million (38 cents), from $5.1 million (10 cents) last year. Revenue increased 30 percent in the quarter, to $317.1 million from $244.2 million a year ago.

The company attributed the increases to improved results in its Canadian construction material operations and from expanded cement activities around the Great Lakes.

W. Bell & Co., a Rockville-based retailer, returned to profitability in its second quarter after a string of losses, the company said yesterday.

W. Bell earned $4.1 million ($1.34) in the quarter, up from a $1.6 million loss a year ago. Part of the profit stemmed from closings of poorly performing stores, which led to a decline in quarterly revenue to $48.9 million from $53.3 million a year ago. For the first six months of its fiscal year, the company said it earned $3.4 million ($1.11), up from a $2.2 million loss a year earlier. First-half revenue fell to $69.1 million from $76.7 million.

Radiation Systems Inc., a Sterling-based maker of antenna systems, said its second-quarter earnings increased slightly, to $1.17 million (22 cents) from $1.16 million (21 cents) a year ago.

The company's revenue fell to $9.5 million during the quarter from $11.3 million a year ago.

For the first six months of its fiscal year, Radiation Systems said it earned $1.7 million (31 cents), down 9 percent from $1.8 million (40 cents) a year ago. During the six months, sales dropped 6 percent, to $18.8 million, from $20 million a year ago.