Two experiments assessed the influence of neonatal hormones on adult sexual behavior in the golden hamster. In Experiment 1 males were castrated or sham-operated on Day 1, 6 or 25 post partum. Sham-operated males were castrated at 40 days of age. Males castrated on Day 1 following birth and receiving adult testosterone propionate (TP) injections failed to show increased mounting toward a receptive stimulus female, while males castrated at later ages (6 days of age or older) responded to adult TP with an augmented mount frequency. Day 1 male castrates were also significantly more responsive to injections of exogenous ovarian hormones (estradiol benzoate and progesterone) than were males castrated at 6 days of age or older. In a second experiment males and females received neonatal injections of either TP or oil and were tested as adults for both masculine and feminine behavior patterns. Females receiving TP on either Day 2 or 4 post partum showed low levels of feminine response patterns (lordosis) and increased mounting behavior. Females receiving TP on Day 10 or oil on Day 2, 4 or 10 post partum showed high levels of lordosis and did not mount. Cyclicity was absent in TP Day 2 and some TP Day 4 females but the remaining females showed 4-day vaginal and behavioral cycles. Both masculine and feminine behaviors in response to exogenous adult hormones were depressed in males receiving TP on Day 2 or 4.