Planning Reasearch Corp., a diversified professional services company, has reported sharply higher earnings for the third quarter ended March 31. Hechinger Co., a retail hardware chain based in Prince George's County, also reported increased profits in the initial quarter of its new fiscal year.

Profits of Washington-based PRC increased 29 percent in the recent quarter to $1.19 million (18 cents a share) from $921,000 (14 cents) a year earlier. Revenues rose by 30 percent to $57.3 million.

PRC President John Toups said the company's backlog of unfilled orders swelled to a record $158 million. And yesterday, PRC said it was awarded a 12-month, $23.6 million contract extension from the National Aeronautics and Space Administration for engineering services.

For the first nine months of its fiscal year, PRC reported net income of $3.36 million (51 cents a share), up 21 percent from the $2.77 million (44 cents) of the previous year. Revenues were $157.8 million, up from $132.6 million.

Hechinger Company's chain of 19 home improvement centers reported a 3 percent increase in sales and a 7 percent gain in profits for the quarter ended April 29.

Although the company said sales were below expectations because of the harsh winter, volume increased from $23.3 million to $23.9 million. Net earnings grew fron $419,000 (16 cents per share) to $447,800 (17 cents).

Along with its earnings, Hechinger announced it has been picked as "Retailer of the Year" among major home improvement chains by Building Supply News, a trade magazine.

Peterson, Howell & Heater, Inc., the Baltimore business services company, reported its annual earnings increased 22 percent last year, from $8.27 million ($1.39 per share) to $10.1 million ($1.67 per share).

Total revenues climbed from $120.8 million to $142.9 million for the firm, which provides personnel and vehicle cost control programs for businesses.

Fourth quarter net income was up 25 percent, from $2.37 million (40 cents per share) to $2.98 million (48 cents per share); quarterly revenues increased from $32.75 million to $42.39 million.

Washington Gas Light Co. reported common stock earnings for the 12 months ended April 30 increased to $13.8 million ($3.19 per share) from $11.6 million ($2.70). Last year's results included an extraordinary item of $1.4 million (21 cents).

The gas company said sales of natural gase declined slightly, from 1.17 billion therm to 1.15 billion therms, but revenues increased from $295.2 million to $352.9 million.

Washington Gas said energy conservation by its customers, warmer weather and a decline in the number of customers served were responsible for the decline in gas consumption.

Radiation Systems, Inc., of McLean reported its third-quarter earnings fell 23 percent, from $142,000 (18 cents a share) to $110,000 (16 cents). The firm's earnings were taxed at a higher level a year ago.

The company said total sales were off 45 percent, from $1.2 million to $658,000. President Harry Letaw, Jr. said the backlog of orders stabilized at $1.1 million during the quarter.

For the first nine months of its fiscal year, Radiation Systems reported sales of $2.4 million, down from $3.5 million, and net earnings of $350,000 (49 cents per share) down from $420,000 (54 cents per share) the previous year when the company had an extraordinary tax credit of $75,000 (nine cents' per share).

Vega Precision Laboratories, Inc., of Vienna, reported earnings in the first fiscal quarter of 30 cents a share compared with 26 cents a year earlier. Sales rose to $3.4 million from $2 million and the company's backlog of orders topped $8 million.