In this study, the following in vitro and in vivo experiments were carried out: (a) Rats were exposed to the radionuclide 57Co2+ ions in single intraperitoneal or intravenous doses (from 10 ng to 1 mg Co/rat) or to 50 ng Co/rat/day through drinking water for 109 days. The target tissue for cobalt depended on the dosage and route of administration (e.g. lung, kidney or bone). Excretion took place mainly through urine (i.p. and i.v. administration) or feces. At 24 h, testis of rats i.v. injected with 10 ng Co/rat contained 0.056% of the dose, with approximately 0.036% in the epididymus, and 0.08% in the deferens. No radioactivity was found in the germinal cells. (b) In vitro incorporation of cobalt in rat sperm. Cobalt enters the germinal cells suggesting that in vivo barriers against the incorporation of the element in sperms may occur. (c) Dose-effect relationships in BALB/3T3 cell cultures exposed to concentrations of cobalt from 1000 to 1 microM of Co2+. Cobalt induced a dose dependent cytotoxic response. At 10 microM, cell growth was reduced to about 30%. No inhibition was found at 1 microM. Morphological transformation assays gave negative results when the cells were exposed to 1000 microM of cobalt.