Riggs National Corp., the holding company for Riggs National Bank, yesterday reported a $4.8 million loss (32 cents per share) for the fourth quarter, compared with profit of $6.7 million (48 cents) in the same quarter a year ago, mainly because of larger reserves to account for potential Third World loan losses.

For the year, the District company said its profits plunged 97 percent to $1 million (7 cents a share) from $35.6 million ($2.65) in 1986.

Riggs said its reserves to cover potential loan losses totaled $66.4 billion at the end of the year, or 2 percent of outstanding loans.

At the end of 1986, those reserves totaled $38.1 billion, or 1.31 percent of outstanding loans. It wrote off as bad $27 million of its loans in 1987, most of them to Third World countries, compared with $5.3 million it wrote off in 1986.

"Our actions in 1987 were consistent with Riggs' conservative approach to asset quality and will help ensure the corporation's long-term growth and stability," said Joe L. Albritton, chairman and chief executive.

Deposits rose to $5.23 billion in 1987, a 4.8 percent increase from $4.99 billion the previous year. Total assets in 1987 rose to $6.79 billion, an 8.5 percent increase from $6.25 billion in 1986, the company said.

Manor Care Inc., a health-care and lodging firm based in Silver Spring, reported that its net income rose 11 percent for the six months ending Nov. 30 to $20.05 million (51 cents per share), from $18.08 million (45 cents).

Revenue rose 4.9 percent, to $266.97 million from $254.32 million in the same period last year.

For the second quarter, Manor Care reported its net income dropped 14.5 percent to $8.9 million (23 cents) from $10.42 million (26 cents) for the same period in the previous year.

Revenue for the quarter rose 5.1 percent to $133.97 million from $127.45 million for the same period last year.

Manor Care operates 144 health facilities, mostly nursing homes, throughout the United States.

Through its Quality Inns subsidiaries, Manor Care also owns, manages or franchises 873 hotels.