The effect of a magnetic field on synovitis in rats was studied. Synovitis was induced by bilateral intraarticular injection of zymosan to the hind joints. One group of rats was held in a cage with a magnet of 3,800 Gauss on the bottom of the cage (treated), while another group was held in a cage without a magnet (control). Three weeks following intraarticular zymosan injection all rats were killed and the hind joints tissues were examined histologically. The pathologic process was noted and graded according to the involvement of various tissue components, number of inflammatory foci, and size of the lesion. The histologic findings of the tissues from the control group consisted of marked articular infiltrate, composed of lymphocytes and plasma cells with some histocytes in 80% of the animals. A small articular infiltration of the same nature was observed in 20% of the treated rats. The inflammatory score was 3.4 +/- 1.1 in the treated animals and 6.8 +/- 2.6 in the control group (P = 0.002). These data showed that synovitis and the inflammatory process are significantly suppressed by a magnetic field.