Strong retail sales and firmness in the research and development industry have led two regional firms, Peoples Drugs Stores Inc. and BDM International Inc., to post increased profits for their most recent accounting periods. But B. F. Saul Real Estate Investment Trust, suffering along with the rest of the real estate business, reported a net loss in the most recent quarter.

Alexandria-based Peoples, one of the nation's leading drug store chains, reported a 14 percent increase in profits for its fiscal year ended Sept. 26. Net income rose to $9.3 million ($2.39 a share) in 1981 from the 1980 fiscal year level of $8.18 million ($2.17).

Sales for the fiscal year increased by 13 percent from $561.8 million to $634.3 million.

Earnings for the fourth quarter totaled $2.46 million (62 cents). For the same time period a year earlier, Peoples' earnings were $2.2 million (57 cents). Sales rose 7 percent to $143.2 million from $133.7 million.

Peoples' top executives noted that even though the company added 43 new stores in fiscal year 1981, its inventories were low intentionally because of high interest costs and the uncertain economy.

BDM and its subsidiaries set record profits and revenue levels for the first nine months of this year and for the latest quarter.

Profits for the third quarter totaled $758,000, a 10 percent increase over the same period last year when the company earned $690,000. Earnings per share decreased, however,, due to an increase in the number of outstanding shares when the company made its first public offering in October 1980. For 1981's third quarter, earnings per share were 34 cents compared with 39 cents for 1980's third quarter.

Revenue for the McLean research and development firm, which specializes in national defense issues, was $22.6 million for the third quarter, a 5 percent increase over the $21.6 million level for the same time period a year ago.

For the first nine months, revenue increased 8 percent from $59.1 million in 1980 to $64.1 million this year. Net income was up 14 percent from $1.9 million to $2.1 million. Because of the increased number of shares outstanding, earnings per share for the first nine months dropped from $1.10 last year to 97 cents this year.

B. F. Saul reported a net loss for the quarter ended Sept. 30 and a decline in earnings for the full fiscal year.

The area's largest real estate trust said a loss in the July-September period of $1.87 million compared with year-earlier profits of $3.2 million (55 cents a share) reflected both a reduction in earnings from the conversion of apartments to condominiums and a sharp boost in short-term borrowing costs.

For the fiscal year, Saul earned $3 million (50 cents) compared with $8.36 million ($1.42) in the previous 12 months. The Saul trust had benefitted in the prior year from an active program of converting apartment buildings it owned into condominium projects, which temporarily increased profits.

In the most recent 12 months, Saul profits from such conversions were $11.6 million compared with $15.9 million a year earlier. Rising mortgage interest rates and a weakening economy resulted in fewer sales in the past year, the company stated. In addition, borrowing costs reduced profits for the year, during which revenues fell to $81.3 million from $84.3 million.

In other reports, Hazleton Laboratories Corp. reported a 41 percent jump in earnings for the three months ended Sept. 30, to $763,000 (23 cents a share) from $542,000 (22 cents). Earnings per share did not keep pace with total profits because the number of shares increased from 2.5 million to 3.4 million.

Revenues of the Vienna scientific testing and research firm increased to $10.9 million from $10.6 million in the same period a year ago.

Systematics General Corp. of Falls Church fell into the red for the latest quarter, reporting a loss of $874,000 for the quarter compared with a profit of $133,000 (8 cents) last year and said revenues fell to $2.7 million from $2.9 million. The firm blamed the loss on "a one-time $613,000 reduction of product inventory costs, unrealized sales projections and high inventory carrying costs."

For the nine months ended Sept. 30, Systematics General reported a loss of $1.2 million on revenues of $9.1 million compared with earnings of $613,000 (45 cents) on sales of $8.4 million in the same 1980 period.