Today a new wave of clinics is opening, driven largely by a motive that was less of a factor in the past: employers’ desires to reduce their health insurance premiums by taking care of workers before they need to see outside doctors. More than 100 of the nation’s 1,000 largest employers now offer on-site primary care or preventive health services - a number forecast to exceed 250 by the end of the year, according to David Beech, a health benefits consultant.

The biggest primary care clinic so far opened Jan. 2 in Texas, when Toyota workers and their families started using a $9 million, 20,000-square-foot medical center alongside a new truck assembly plant in San Antonio. Unlike most of the new medical offices — which are staffed by nurse practitioners and in some cases a part-time doctor — Toyota’s San Antonio health center has two-full time doctors, a part-time physician, a blood-test lab and an X-ray center.

Other companies have eliminated primary-care clinics, either because they did not consider them cost-effective or for other reasons. Ford Motor, which used to offer basic health care in the 1950s at its big Rouge plant in Dearborn, Mich., now says it believes that most employees prefer to go to their own doctor for primary care.

But the foreign-based automakers in the United States, including Nissan and BMW, which do not have union-negotiated benefits or large numbers of retirees, are embracing the on-site health trend for their work forces. Comprehensive Health Services recently announced plans to provide clinic services at three existing Nissan plants and the company’s new North American headquarters in Franklin, Tenn.