Communications Satellite Corp. reported yesterday that operating profits declined in the fourth quarter of 1981 and for the full year, although an accounting change boosted net income of the D.C.-based firm in the recent 12 months.

Separately, two major research and technology firms--Atlantic Research Corp. and Hazleton Laboratories Corp.--posted sharp increases in profits, while Washington Real Estate Investment Trust said earnings rose 30 percent in 1981, and B.F. Saul REIT reported a $1.18 million loss for its latest three-month period.

Comsat, a diversified communications firm, said fourth-quarter profits fell to $7.4 million (92 cents a share) from $9.7 million ($1.22) in 1980 as revenues rose to $89.5 million from $82 million.

For the full year, operating profits fell to $28.3 million ($3.53) from $38.3 million ($4.79) in 1980 as operating revenues rose to $334 million from $300 million. In 1981, a cumulative change in accounting for investment tax credits added $11.8 million ($1.47) and boosted net income to $40 million ($5).

Comsat's expenses for new businesses soared last year, accounting for the decline in operating profitability. Operating and maintenance expenses jumped a planned $32 million. Research spending rose $1.5 million to $15.2 million.

In addition, Comsat's share of losses in the fledgling Satellite Business Systems domestic communications company rose from $12.4 million in 1980 to $21.1 million last year, after taxes. SBS, owned by Comsat, International Business Machines Corp. and Aetna Life & Casualty Co., is expected to be a major business communications venture.

Atlantic Research of Alexandria said profits rose 52 percent in 1981 to a record $3.5 million ($1.90) from $2.3 million ($1.36) the previous year as sales increased 16 percent to $93.2 million. Fourth-quarter profits were $986,000 (54 cents) vs. $640,000 (35 cents) and sales rose to $25.8 million from $24.9 million.

ARC, a major missile development and manufacturing firm, also announced completion of a 75-barrel-a-day pilot line for a coal-water slurry project to create synthetic fuel.

Hazleton Laboratories, a Vienna-based scientific company, said profits rose 17 percent in the second quarter ended Dec. 31 to $738,000 from $631,000 a year earlier. Per-share earnings were 22 cents in both periods, because of 19 percent more common shares outstanding.

For the first six months of the firm's fiscal year, profits rose 32 percent to $1.53 million (45 cents) from $1.16 million (43 cents) but sales were flat at $22 million.

Washington Real Estate Investment Trust's earnings last year were a record at $4.5 million (95 cents a share) compared with $3.5 million (76 cents) as total revenues rose to $13.5 million from $12.4 million. In the final quarter, WRIT, which owns apartments and office buildings on which rents grew, earned $1.3 million (27 cents) vs. $1 million (23 cents), and revenues rose to $3.5 million from $3.2 million.

WRIT voted a quarterly dividend of 25 cents a share on March 31, to owners of record March 5.

B.F. Saul Real Estate Investment Trust listed an operating loss of $1.18 million for its first fiscal-year quarter ended Dec. 31 compared with year-earlier profits of $2.9 million (50 cents). When one-time tax credits are added to both periods, the net loss in the recent quarter was $1.1 million compared with net income of $4.2 million (71 cents). Cash flow from operations was a deficit of $176,000 compared with a gain of $6.9 million a year ago. Most of the Saul swing reflects an unusually large number of condominium conversions at apartment complexes a year ago, which temporarily boosted earnings.