This study examined the effects of acute exposure to PCB99 (2,2',4,4',5-pentachlorobiphenyl), and PCB153 (2,2',4,4'5,5'-hexachlorobiphenyl), on spermatogenesis in 8-week-old C57BL6 mice. The mice were randomly allocated to PCB99 and PCB153 and a single dose of respectively 10 and 100 mg/kg was given by oral gavage. During the 6-week experiment, six mice per treatment group were sacrificed weekly, body weights were recorded and samples with respect to the male reproductive system were collected until further analysis. None of the treatments, showed changes in body weight or reproductive endpoints. Flow cytometric analysis revealed spermatogenesis to be unaffected. However, PCB99 and PCB153 showed a significant increase in Leydig cell apoptosis. The results from the present study indicate that the male reproductive system is relatively refractory to PCB99 and PCB153 at levels exceeding those of wildlife and humans, when exposed during adult life. However, the finding of apoptotic Leydig cells merits further investigation.