Cod were fed herring containing Aroclor 1254, a polychlorinated biphenyl (PCB), at diet levels of 0, 1, 5, 10, 25, and 50 microgram/g for a period of 5 1/2 months. Histological examinations of the gonads of surviving male fish revealed various testicular abnormalities in 9 of 17 PCB-fed fish but in none of four experimental control and four stock control fish. The abnormalities were observed in testes that were either at functional maturity or in a stage of rapid spermatogenic proliferation but not in testes that were sexually immature or regressed. The testicular abnormalities included disorganization of lobules and spermatogenic elements, inhibition of spermatogenesis, fibrosis of lobule walls, fatty necrosis, and, in one case, total disintegration of the elements in many lobules. There was a significant uptake of PCB by testicular and liver tissues of fish that were fed the higher levels (greater than 1 microgram/g) of Aroclor 1254.