Primedical Corp., an Annapolis-based chain of walk-in health clinics, has been acquired for an undisclosed amount by one of its largest competitors -- Health Stop Medical Management of Wellesley, Mass.

Primedical's 67 medical offices across the country will be added to Health Stop's network of 62 clinics to make the largest provider of walk-in service in the country.

Health Stop officials, whose privately held firm also includes hospital emergency physicians services, physician billing programs and occupational health care, predict revenue for 1988 of more than $80 million.

Joseph G. Maloney, executive vice president of Health Stop, said that when the purchase becomes final next month, Primedical's Annapolis headquarters will be closed and some of its corporate staff laid off, although the newly acquired clinics will retain the Primedical name.

Until nine months ago Primedical was a small, seven-clinic company. But it suddenly became a major health-care provider by agreeing to buy 67 walk-in clinics from Humana Inc., the Louisville-based, for-profit hospital chain. The Humana centers, which were scattered in eight regions of the country, were unprofitable, but Primedical management called their purchase the "deal of a lifetime" and vowed to turn them around.

"We can reduce overhead and be more flexible," Primedical President Stanford Ellsworth said at the time.

But Health Stop officials said that Primedical continued to lose money, and the deal between the two companies -- which will take the form of a stock swap -- was made because of Primedical's need for management expertise.

"They made a good effort, but there is a lot to learn in this business," said Maloney. "We are in a position where we can add some value to Primedical by using {our management} systems in their office, reimbursement, staff management, marketing, accounting and financial operations."

Primedical officials could not be reached for comment.

"We're taking on a losing operation on the premise that we are going to be able to make the necessary changes," said Maloney.