Dietary cobalt (265 ppm Co) induced polycythemia and consistent degenerative and necrotic lesions in the seminiferous tubules of rats. Cyanosis and engorgement of testicular vasculature on day 35 and thereafter was followed on day 70 by degenerative and necrotic changes in the germinal epithelium and Sertoli cells. Spermatogonia, primary spermatocytes and round spermatids were markedly affected, while elongated spermatids, spermatozoa, and sertoli cells were more resistant. Damaged tubules, often present side by side with normal tubules, contained multinucleated giant cells composed of degenerated and necrotic spermatocytes and/or spermatids, sloughed germinal and Sertoli cells, and calcified necrotic debris. Necrotic tubules were frequently collapsed and devoid of epithelium except for occasional spermatogonia and surviving Sertoli cells. Lesions were not observed in the Leydig cells, cauda epididymis or seminal vesicles.