RCA Corp. yesterday reported the highest earnings of any quarter in the company's 59-year history in the second quarter with a 12 percent gain over a year ago.

Profit was $78.3 million, or $1.02 a share, up from $70.1 million, or 92 cents a share, a year ago as sales rose to $1.61 billion from $1.43 billion.

First-half profit was $138.2 million, or $1.74 a share, on sales or $3.12 billion from $118.6 million, or $1.55 a share, a year ago on sales of $2.8 billion.

President Edgar H. Griffiths said the gains were accomplished in spite of the fact that National Broadcasting Co. profit was soft and is expected to be down from last year for all of 1977. But consumer and commercial electronics, the Hertz vehicle rental division, government business and the communications network all had excellent gains. Hertz had the biggest profit gain for the quarter, up 26 percent.

General Electric Co., the nation's largest and most diversified producer of electrical and electronic equipment, yesterday reported net earnings of $319.4 million, or $1.40 a share, for the second quarter, an increase of about 17 percent from the $271.9 million, or $1.20 a share, reported in the same period last year.

Sales for the Fairfield, Conn.-based firm rose 13.2 percent to $4.96 billion for the quarter from $4.38 billion last year, the company said.

The company attributed the gains to strong performances in all segments of the business, particularly in the areas of consumer products, construction and transportations systems, and power generators.

The Caterpillar Tractor Co. has announced second-quarter sales and profits were up more than 25 percent a record for one-quarter increases for the Peoria-based firm.

Increased volume, including shipments of a "large number of D9 pipelayers to the Soviet Union, was the main factor in sending second-quarter sales up 26.7 percent, to $1.84 billion, the company said.

Second-quarter profits increased 28.3 percent, to $150.2 million. If the trend continues through 1978, Caterpillar sales would reach $7.1 billion, compared with $5.8 billion last year.

As a result, the firm's worldwide employment has grown by more than 6,000 during the last 12 months, Caterpillar said.

International Paper Co. earned $1.46 a share in the second quarter, up from $1.26 a year ago, on an 11 percent sales gain to $1.05 billion from 946 million.

First half profit was $127.2 million or $2.69 a share on sales of $2.01 billion compared with $107.7 million or $2.30 a share on sales of $1.8 billion.

Chairman J. Stanford Smith said newsprint, corrugated packaging containers and industrial papers all continued to the gains. Smith said he was especially pleased by the substantial reduction during the half in world commodity pulp inventories, particularly those in Scandinavia and Canada, because this augured better operating conditions to come.

International Paper had a loss on foreign currency translations of $2.4 million in the second quarter compared with a loss of $1.6 million a year earlier.

Georgia-Pacific Corp., the timber products concern, yesterday reported record sales and earnings for the second quarter and first half of 1978.

In the second quarter, the company earned $85 million, or 82 cents a share, up 25 percent from the $68 million, or 66 cents a share, in the corresponding 1977 quarter. Sales climbed 21 percent to $1.34 billion from $939 million in the year-earlier period.

For the first six months, net income rose 20 percent to $149 million, or $1.44 a share, from $124 million, or $1.20 a share, in the 1977 first half. Sales totaled $2.022 billion, a gain of 18 percent over the $1.719 billion recorded in the corresponding period last year.

Robert E. Flowerrae, chairman and chief executive of the Portland, Ore., firm, said demand for building products was at "vigorous levels" in the second quarter, while chemical markets remained steady.

Marine Midland Banks, Inc., said a substantial gain in operating profit and a $1.45 million tax credit enabled it to report a 57 percent gain in per share earnings for the second quarter.

The gain occurred in spite of a $371,000 loss on securities transactions. Profit amounted to $6.61 million or 52 cents a share against $3.97 million or 31 cents a share a year earlier.

First half profit was $10.83 million or 86 cents a share a year ago. Of the share profit, the tax credit accounted for 11 cents a share in both the quarter and the half.

The bank reduced the volume of loans on which it was not being paid interest to $344 million during the half compared with $410 million a year earlier, but the provision for loan losses rose to $13.1 million from $12 million a year ago.

Girard Corp., parent of the Philadelphia-based Girard Bank, earned $1.07 cents a share in the second quarter, up from 92 cents a year ago. The operating earnings of $1.14 a share were reduced by losses on securities transactions.

Profit for the half was $1.92 a share, up from $1.65 a year ago. Net income for the quarter was $5.41 million, up from $4.51 million a year ago. Net for the half was $10.66 million, up from $9.0 million.

Chairman William B. Eagleson Jr. said higher interest rates were responsible primarily for the gains.

Whirlpool Corp., producer of home appliances, said sales and earnings for the first quarter and six months were sharply higher than in the 1977 periods.

Company Chairman John Platts said the first half gains were "especially impressive" in view of production interruptions and a shortage of equipment stemming from the severe winter weather.

For the first quarter the company earned $34.4 million or 95 cents a share against $27.65 million or 76 cents a year earlier. Sales moved up to $619.13 million from $507.64 million.

For the half the company earned $59.61 million or $1.65 a share compared to $52.51 million or $1.45 in the first 1977 half. Sales totaled $1.10 billion against $948.15 million a year ago.