Effects of cobalt on murine testes were evaluated by light and electron microscopy. Continuous exposure of male mice to cobalt (400 ppm) via drinking water over a 13-week period resulted in a reproducible, sequential pattern of seminiferous tubule degeneration. Initial changes involved vacuolation of Sertoli cells and formation of abnormal spermatid nuclei. This was followed by the presence of multinucleated cells and sloughing of cells. Sertoli cells phagocytosed degenerating cells. Continued degeneration resulted in shrinkage of tubules with the accumulation of "calcified" necrotic debris in some. Sertoli cells were the last surviving cells. As degenerating tubules shrank, peritubular areas became highly disorganized. Myoid cell shapes became irregular, and basal laminae became highly folded. Endothelial cells of testicular vessels were thickened in areas and contained vesiclelike structures. Leydig cell morphology was normal but interstitial areas appeared hypercellular. The possible interactions of cobalt with iron and zinc, essential metals for spermatogenesis, are discussed.