The mild summer and warm winter last year helped reduce the profit of Potomac Electric Power Co., the District-based utility said yesterday.

Pepco said its net income for the year fell 21 percent to $170.2 million ($1.62 per share) from $214.6 million ($2.16) in 1989. Revenue rose slightly, to $1.41 billion from $1.39 billion.

The company said the summer temperature, as measured in cooling degree hours, was 12 percent below the area's 20-year average and 1990's winter weather was the mildest in 20 years. In addition to the weather, Pepco said its earnings were affected by a regulatory delay in recognizing the cost of power it purchases and lower profit at its non-utility subsidiary, Potomac Capital Investment Corp.

In the fourth quarter, Pepco's profit fell 46 percent to $19.5 million (17 cents) from $35.9 million (35 cents) in the same period a year earlier. Revenue rose 4 percent to $306.9 million from $295 million a year earlier.

LaFarge Corp., the Reston-based construction materials company, said that the building slowdown halved its profit last year.

LaFarge, whose cement division is the nation's second-largest supplier of that material, said its net income for the year fell 50 percent to $50 million (97 cents a share) from $100 million ($1.98) in 1989. Revenue rose 7 percent to $1.6 billion from $1.5 billion the year before.

In the year's final quarter, the slide was steeper. LaFarge's net income fell 67 percent to $6.7 million (13 cents) from $20.4 million (40 cents) in the same period a year earlier. Revenue fell 1 percent to $390.2 million from $394.2 million a year earlier.

"Our earnings performance reflects the relatively low level of construction starts in several of our major markets during 1990," President Robert W. Murdoch said. "Volumes and margins in central Canada and the Northeast U.S. have dropped off significantly, and we have not seen evidence to suggest a turnaround in building activity at this point in time."

Radiation Systems Inc., the Sterling-based antenna maker, said its profit rose 54 percent in the quarter ended Dec. 31.

Net income in the quarter, the second of the company's fiscal year, was $2 million (36 cents a share), compared with $1.3 million (24 cents) in the same quarter a year earlier. Revenue rose 62 percent to $28.1 million from $17.4 million a year earlier.

The company also said that its backlog of unfilled orders was at a record level, $181.4 million at year-end, compared with $126.8 million a year earlier.