In a series of six experiments, the experiential and hormonal factors influencing infanticide in Long-Evans hooded rats were examined. Males, but not females, purchased from a breeder as adults exhibited infanticide at high rates, whereas males reared in the laboratory until adulthood were less likely to be infanticidal. Sexual experience reduced the rate of infanticide in laboratory-reared males, and cohabitation with a female through pregnancy and lactation (including exposure to pups) inhibited infanticide in previously infanticidal males. Fourteen days of cohabitation with a pregnant female following copulation inhibited infanticide in males purchased from a breeder, but copulation without cohabitation and copulation followed by cohabitation with a nonpregnant (ovariectomized) female did not inhibit infanticide. Castration of laboratory-reared males in adulthood did not reduce their rate of infanticide, whereas testosterone implants increased the number of males exhibiting infanticide. The importance of hormonal and experiential factors for controlling infanticide in male rats is discussed.