Developmental effects of PCBs on male reproduction in rodents have been studied previously in several laboratories, however, with inconsistent results. The effects on testicular weights and fertility depend on the test congener or mixture, the dosage, the developmental stage during exposure, and the age of the animals when examined. We have previously reported an increase in testis weight and a decrease in sperm fertilizing ability in 3,3',4,4'-tetrachlorobiphenyl treated young males. The effect of Aroclor 1254 on sperm fertilizing ability after neonatal exposure, however, was not observed until 45 weeks of age. The objective of this study is to examine the effects of gestational and lactational exposure of Aroclor 1242 (0, 10, 25, 50 and 100 mg/kg/2 days) on testicular development and sperm quality in 16-week old male mice. Sperm concentration and quality were examined using in vitro fertilization and computer-assisted sperm motion analysis. Testicular gene expression in mice from 25mg/kg treatment group was examined using a commercial cDNA array. There was no effect on maternal body weight, fecundity, litter size, litter weight, sex ratio or 21-day survival index. There was a dose-dependent increase in liver weights in 21 day old male offspring (p < 0.04), but no observable change in testis and thymus weights at 3 weeks of age. There was, however, an increase in anogenital distance in the male offspring in the 25 mg/kg treatment group on day 21 (p < 0.002). At 16 weeks of age, there were no detectable changes in sperm count or fertilizing ability. There were no changes in liver or thymus weight, however, there was a dose-dependent trend of increased testes weight. cDNA array analysis did not reveal any changes in gene expression. (Sponsored by the Michigan Great Lakes Protection Fund).