The present study examines whether genetic male and female gerbils differ in response to neonatal steroid treatment as evidenced by the display of sexually dimorphic territorial marking behavior in adulthood. The advantage of this behavioral endpoint is that male and female marking differs only in frequency, permitting direct between-sex comparison of neonatal treatment effects. Females and day-2 castrated males received a single SC injection of 10 to 100 micrograms testosterone propionate (TP) or estradiol benzoate (EB) between birth and day-14 postpartum. Between 7 and 9 months of age, the territorial marking behavior response to exogenous TP was determined in all animals. Based on marking frequency, the time of greatest sensitivity to TP administered during the critical neonatal period occurred earlier in the male (days 3-5) than in the female (days 5-7). In addition females consistently showed greater sensitivity to a given dose of either TP or EB than did males. EB was more effective than TP at the 10 micrograms dose in both sexes. These results indicate that sex differences exist in timing and steroid sensitivity in the sexual differentiation of the neural substrate regulating territorial marking behavior.