Geico Corp., the holding company that owns Government Employees Insurance Co., reported yesterday a sharp upturn in operating income from its insurance business, but a slight downturn in profits for the three months ended June 30.

While Geico's insurance income climbed to $16.5 million from $11.7 million, net profits fell to $20.5 million from $20.9 million, largely because the company did not get as much in tax credits from previous losses.

On a per share basis, however, Geico's profits went up, to 79 cents per dhare from 59 cents, because the company repurchased stock and exchanged debentures for shares earlier this year.

For the first six months of the year, Geico's operating income was about the same as the previous year - $29.6 million, versus $29.2 million - but net declined to $40.3 million from 441.5 million. Per share profits for the first half increased to $1.35 from $1.17.

Heritage Financial Corp., of Richmond, listed earnings of $702,300 (37 cents) vs. $775,900 (41 cents) as assets rose $40 million to $247 million.

Baltimore Federal S&L earnings declined 3 percent to $3.07 million as assets increased $69 million to $1 billion.

Overnite Transportation Co. profits rose in the second quarter but earnings of another trucking firm, Preston Trucking Co., declined. Overnite, based in Richmond, said profits rose 47 percent to $5.5 million ($1.72 a share) from $3.8 milltion ($1,18) as revenues rose 21 percent to $61 million. Preston profits fell to $977,000 (31 cents) vs. $1.9 million (62 cents) as revenues dipped 0.2 percent to $38 million. A strike in April curtailed operations that month at Preston, based in Preston, Md.

Maryland Realty Trust reported a net loss of $52,380 in the second quarter ended May 31 compared with profits of $126,225 (17 cents a share) in the 1978 period.

Virginia Chemicals Inc. reported record second-quarter profits of $1.75 million (69 ceents a share) compared with $1.56 million (64 cents) as sales rose 16 percent to a record $34 million.

C3 Inc., a Reston manufacturer of minicomputer products, reported earnings of $1.7 million for the year ended March 31 compared with $1.5 million the previous year. Sales rose to $10.7 million from $7.5 million.