Sovran Financial Corp., the region's largest bank holding company, yesterday reported a 16.8 percent rise in earnings for the first quarter, largely from an increase in interest income on loans and other assets.

During the quarter, the Richmond bank holding company completed a merger with Suburban Bank in Bethesda and D.C. National Bank, making it the first company to complete interstate acquisitions in two jurisdictions under newly enacted regional interstate banking laws.

The company's net income for the first three months of the year was $34.3 million (82 cents a share), up from $29.4 million (70 cents) in the same period in 1985.

Sovran Chairman C. A. Cutchins III said profits were dampened by increased costs associated with the mergers. That helped push down the company's return on average assets to 1.07 percent, from 1.09 percent in the first quarter of 1985.

Sovran wrote off $5.2 million in loans in the first quarter, up from $4.1 million last year. Reserves against future write-offs also increased, to $106 million at the end of the quarter from $83.4 million at the end of the first quarter last year.

*Washington Bancorp., the third-largest bank holding company in the District, reported an 8 percent increase in earnings for the first quarter of 1986.

The company, which owns The National Bank of Washington, said earnings were $1.64 million ($1.64 a share) in the first three months of 1986 compared with $2.4 million ($1.70) for the same period last year. An increase in the number of shares outstanding caused the drop in per-share earnings.

Because assets increased faster than earnings, however, the company's return on assets declined to 0.72 percent in the first quarter from 0.75 percent in the same period last year.

Average assets, loans and deposits increased slightly: Assets were $1.4 billion, up from $1.3 billion; loans were $900 million, up from $800 million; and deposits grew to $1.1 billion from $1 billion.

*Syscon Corp., a Washington, D.C., computer company catering primarily to government customers, yesterday reported earnings increased 12 percent in the first quarter of fiscal 1986 on a 12 percent increase in sales.

The company said earnings for the quarter that ended Feb. 28 were $1.1 million (24 cents a share) compared with $986,000 (21 cents) in the same period last year.

Revenue for the period was $29.2 million compared with $26.2 million. The company said it expects the record results to continue in 1986.

*Ryland Group Inc., one of the nation's 10 largest home builders, reported that earnings jumped 201 percent in the first quarter of 1986 on a 54 percent increase in revenue.

The Columbia, Md., company said it earned $2.6 million (40 cents a share) on $120.3 million in sales in the first three months of the year. For the same period last year, the company earned $848,000 (14 cents) on revenue of $78.3 million.