Phenobarbital, a potent antiepileptic drug, was tested for the induction of dominant lethal mutations in male Swiss mice. The drug was administered in sterile distilled water at doses of 42, 84 or 126 mg/kg body weight by intubation over five consecutive days. Controls were given distilled water alone. The males were then mated for 1 wk sequentially with pairs of untreated virgin females for a total of 8 wk. The females were killed 17 days after presumed mating and the uterine contents were examined. There was a significant reduction in the total number of implantations and an increase in the number of dead implantations per pregnant female in the treated groups compared with the controls. The results from sequential matings showed that the pre-meiotic stages of spermatogenesis were more affected than the post-meiotic stages.