Many species that engage in parental behavior exhibit infanticide under certain circumstances. The neural signals regulating the transition from infant care giver to infant killer and back remain unclear. Previously we demonstrated that progesterone (P) and its receptor (PR) have inhibitory effects on parental behavior and increase infant-directed aggression in male mice. In the present studies we sought to elucidate the mechanisms by which the effects of P are manifested. Because the onset of parental behavior in females is associated with the withdrawal of P at the end of pregnancy we tested the hypothesis that withdrawal of P would similarly enhance parental behavior in males. Virgin male mice were implanted with P or vehicle for 21 days, replicating the duration of pregnancy in females. Tests were run for parental and infanticidal behavior 5 days after removal of the capsules. P increased the proportion of nonparental males that attacked pups. However, neither the number of males exhibiting parental care nor the quality of care was affected by P treatment. Serum P and testosterone (T) levels were not different from controls at the time of behavioral testing indicating continued elevations in peripheral hormones are not required for the expression of infanticide. In conclusion, withdrawal of P does not trigger the onset of parental behavior in males. Rather, prior exposure to P induces persistent infanticidal behavior in adult male mice.