Manor Care Inc. reported a 33 percent increase in profits for its latest fiscal quarter and attributed the improvement to recent expansion of its nursing center operations through acquisition and internal development.

Also posting increased profits were Potomac Electric Power Co., Circuit City Stores Inc., Penril Corp., U.S. Health Inc., and Hotel Investors Trust and Hotel Investors Corp., but Biotech Research Laboratories Inc. reported a narrower loss than a year ago and Stanwick Corp.'s earnings declined.

Manor Care, a health-care and lodging company with headquarters in Silver Spring, reported net income of $7.8 million (29 cents a share) for the three months ended Nov. 30, up 33 percent from $5.9 million (22 cents) in the same period last year. The earnings reflect a three-for-two stock split distributed Dec. 6.

Quarterly revenue increased to $112.7 million from $89.8 million in the same period of 1983.

For the six months ended Nov. 30, Manor Care reported profits of $15.4 million (58 cents), up 31 percent from $11.8 million (45 cents). Six-month revenue grew to $175 million from $132.4 million. * Potomac Electric Power Co. reported net income of $168.3 million ($3.23 a share) for its fiscal year ended Nov. 30, up 20 percent from $140.2 million ($2.67) the year before. The utility reported total operating revenue of $1.19 billion compared with $1.17 billion the year before.

* Biotech Research Laboratories Inc. reported a third-quarter loss of $201,177 (4 cents a share) compared with a loss of $284,067 (6 cents) in the same period last year. Revenue increased to $888,889 from $464,734.

The Rockville company also reported a nine-month loss of $758,418 (15 cents) on sales of $2 million compared with a loss of $881,477 million (17 cents) on sales of $1.3 million.

In October, Biotech Research and E. I. du Pont de Nemours & Co. jointly supplied for clinical trials their first testing kits to detect AIDS (acquired immune deficiency syndrome). The companies plan to begin marketing the kits in early 1985 once the trials are complete.

* Circuit City Stores Inc. reported third-quarter earnings of $3.9 million (38 cents a share), up 52 percent from $2.6 million (26 cents) the year before. Revenue for the quarter rose to $137.3 million from $94.4 million last year.

The Richmond-based consumer electronics retailer said that nine-month profits rose 80 percent to $11.4 million ($1.11) from $6.3 million (65 cents) during the same period of last year. Revenue increased to $339.5 million from $232.1 million.

Circuit City operates 47 audio and electronics stores from New York to Georgia, 13 warehouse showrooms and other outlets.

* Penril Corp. of Rockville reported profits of $770,000 (36 cents a share) for its fiscal first quarter ended Oct. 31, up 63 percent from $473,000 (23 cents) the year before.

The company, which manufactures computer data communications equipment, electronic instruments and audio systems, said its sales doubled to $21 million from $10.5 million the year before.

The results include about $8.4 million of revenue contributed by Penril's two acquisitions last year, the company said.

* U.S. Health Inc. reported first-quarter profits of $792,000 (15 cents a share), up 34 percent from $592,000 (11 cents) in 1983. The Towson-based company's revenue increased to $11.4 million from $8.7 million.

The firm is a holding company that operates Holiday Spas and Fitness Centers in the eastern United States and provides accounting, administrative and collection services.

* Hotel Investors Trust and Hotel Investors Corp. of Chevy Chase reported combined first-quarter net income of $1.7 million (65 cents a share), up 48 percent from $1.15 million (44 cents) for the same period of last year.

Revenue increased to $9.4 million from $6.4 million the year before.

Hotel Investors Trust is an equity-oriented real estate investment trust that specializes in the hotel and accommodations industry. The trust owns or finances Marriott, Hilton, Sheraton, Ramada and other hotel properties. Hotel Investors Corp. leases hotel properties from the trust and manages hotels for other owners.

* Stanwick Corp. reported a 96 percent drop in second-quarter net income to $1,403 (no cents a share) from $36,400 (4 cents) in the same period of 1983. Revenue increased to $1.2 million from $1 million last year.

For the six months ended Oct. 31, net income increased 388 percent to $196,162 (19 cents) from $40,230 (3 cents) the year before. Sales for the six months increased to $2.7 million from $2.3 million the year before.

The District-based company blamed the quarterly decline on increased marketing and business development costs associated with the company's expansion programs and high nonrecurring costs associated with a modification of one of the company's major contracts.

Stanwick provides maintenance management, technical support, integrated logistic support, and systems engineering and management services.