Riggs National Corp., the largest bank holding company in the District, yesterday said its profits rose 69.7 percent in the first quarter of the year, mostly from gains on the sale of long-term government securities.

Net income for the first three months of 1986 was $12.4 million ($1.03 a share) compared with $7.3 million (61 cents) for the first quarter of 1985.

Interest income from loans, securities and other instruments increased 1.8 percent from the first quarter of 1985 to $28.8 million.

Noninterest income -- such as service fees and investment profits -- was $21.8 million, an increase of $13.6 million, or 165 percent, from the $8.2 million earned from that source during the same period last year. Nearly all of the gain -- 94 percent -- came from a $12.8 million profit on the sale of government securities.

The company's major holding is Riggs National Bank. The bank's deposits increased 10 percent to $4.25 billion during the first quarter of 1986, while nonperforming assets declined substantially, to $6.7 million from $25 million in the same period last year.

The bank's loan loss reserve, however, increased to $42.2 million, or 1.59 percent of loans, from $32.1 million, or 1.55 percent of loans, the company said.

*CSX Corp., the nation's second-largest railroad, reported a 5 percent drop in earnings on a 15 percent drop in revenue during the first three months of 1986.

The Richmond-based railroad company, which also has energy, technology and property divisions, said it earned $85 million (56 cents a share) on revenue of $1.7 billion during the first quarter. That compares with profits of $90 million (60 cents) on revenue of $2 billion during the first quarter of 1985.

The company blamed the quarterly decline on a drop in land sales. Most of the property being sold is surplus railroad property. Rail competition also hurt the company, cutting its transportation revenue 5 percent even though traffic volume during the quarter remained constant from last year.

The revenue decline was only partly offset by a 17 percent cut in operating costs, the company said.

The company said that any drop in revenue from the fall of oil prices during the next three quarters will likely be offset by a drop in the price of the diesel fuel that is used by its locomotives.

*Data Measurement Corp. of Gaithersburg reported net income of $328,468 (90 cents a share) for 1985, a 64 percent gain over $200,545 (55 cents) the year before. Revenue increased to $5.6 million from $4.6 million. The company said the results reflect its progress as a maker of measurement and control systems for basic industries.