Female hamsters were treated with oil, 3-mug., 30-mug., or 300-mug. testosterone propionate (TP) as neonates. Neonatal TP treatment delayed the onset of puberty by 4.5 days to an age near that previously reported for the male hamster. In addition, neonatal TP altered genital morphology, induced the capacity for mounting behavior, and at the highest dosage, disrupted the ability to bear and rear young. Vaginal and behavioral estrous cycles, however, were not influenced by neonatal TP. In a second experiment, 600-mug. TP administered neonatally blocked estrous cyclicity, but did not eliminate the capacity to display feminine sexual behavior. These results imply that "masculinization" and "defeminization" are separate aspects of neurobehavioral sexual differentiation, and the "defeminization" includes several independent physiological processes.