The Rouse Company, the Columbia, Md.-based retail development firm, reported increased second-quarter and six-month earnings.

Net earnings for the second quarter were $919,000 (6 cents), compared with $702,000 (5 cents). Revenues were $32.7 million, up from $28.2 million.

For the first half, net earnings were $1.6 million (11 cents), compared with $1.3 million (9 cents) for the first six months of 1980. Revenues were $63.2 million, up from $56.2 million.

All of the firm's operating divisions showed improved results during the second quarter, Rouse President Mathias J. DeVito said in a report to shareholders. "For the six months only the mortgage banking division trailed last year, and this should be reversed by year end," DeVito said.

"Four development projects had very successful openings, and a number of others made good progress in the development pipeline. While the prospect of continued high interest rates gives us concern, we remain very optimistic about 1981 and the company's future in the years ahead," DeVito said.

B.F. Saul Real Estate Investment Trust reported a third-quarter loss of $380,000, compared with a net profit of $2.5 million (43 cents) a year earlier. During the latest quarter a tax credit of $28,000 made the final net loss $352,000. During the same quarter last year a tax credit of $2.1 million made the final net profit of $4.6 million (78 cents).

For the past nine months the operating net profit was $3.1 million (53 cents), compared with $2.9 million (49 cents). This year a tax credit of $1.7 million made the final net profit $4.9 million (82 cents), while a tax credit of $2.3 million the year earlier made that final net profit $5.1 million (87 cents).

C3 Inc., a Reston computer equipment firm, reported a 65 percent increase in first-quarter earnings.

Net income was $1.8 million (21 cents a share), compared with $1.1 million (15 cents). Revenues increased 57 percent to $10.7 million from $6.8 million.

Per-share earnings figures were adjusted for a two-for-one stock split effective June 30.