Chronic exposure of male mice to cobaltous chloride dramatically affected their reproductive potential, while acute administration had minimal effects. Acute exposure, followed by evaluation weekly over a 7-week period, revealed no significant changes in epididymal sperm concentration or testicular weight. However, small but significant decreases in fertility at weeks 2 and 3 of the study were observed. Sperm motility was depressed only during the first week of the study. In chronic studies, cobalt affected fertility in a time- and dose-dependent manner. There was a decrease in testicular weight, epididymal sperm concentration, and fertility. Sperm motility was also depressed. Serum testosterone levels were dramatically increased in cobalt treated animals, while FSH and LH serum levels were normal. It appears that cobalt is directly or indirectly interfering with spermatogenesis and with local regulatory mechanisms in testosterone synthesis.