The purpose of the present study was to investigate histologically and ultrastructurally the effect of adriamycin on the mandibular condyles of growing rats. Eighty rats, one day old, were used. The animals of Group 1 (Control Group) were given distilled water once a day for five days by intraperioneal injections, and the animals of Group 2, Group 3 and Group 4 were given 0.6 mg, 0.9 mg and 1.2 mg of adriamycin/kg (total amount: 3 mg/kg, 4.5 mg/kg, 6 mg/kg), respectively, once a day for five days. At 1, 4, 7 and 10 days after the last injection, the left mandibular condyles were examined histologically. At 1 and 4 days, the right mandibular condyles were examined ultrastructurally. 1. Light Microscopic Observation. 1) Group 2. At 1 and 4 days, the condyles did not differ significantly from those of Group 1. At 7 and 10 days, the zone of differentiation decreased in thickness as compared with those of Group 1. 2) Group 3. At 1 and 4 days, the zones of articular surface, proliferation and differentiation showed decreased in cell number and in thickness as compared with those of Group 1. At 7 and 10 days, the zones of articular surface and proliferation showed the recovery, although it was not complete. However, the zone of differentiation still decreased in cell number and in thickness. The zone of hypertrophy showed an increase in thickness. 3) Group 4. At 1 and 4 days, the zones of articular surface, proliferation and differentiation were almost similar in cell number and in thickness to those of Group 3 at 1 and 4 days. There was an irregularity in cell shape in the zone of proliferation. At 7 and 10 days, the zone of proliferation decreased in thickness and the zone of differentiation decreased in cell number and in thickness. The zone of hypertrophy was greatly thinned. 2. Electron Microscopic Observation. 1) Group 1. At 1 day, the zone of articular surface was composed of fibroblasts and collagen fibrils, and the zone of proliferation contained many oval mesenchymal cells. The zone of differentiation consisted of both differentiating chondrocytes and cartilage matrix, and the zone of hypertrophy consisted of both enlarged chondrocytes and cartilage matrix. At 4 days, all zones were similar to those of the condyle of this group at 1 day. 2) Group 4. At 1 day, the zone of articular surface showed decreased number of collagen fibrils.(ABSTRACT TRUNCATED AT 400 WORDS)