Sex differences have been observed in the perinatal sexual differentiation of the neural substrate which regulates territorial marking behavior in the Mongolian gerbil. The present study examines the relative contribution of prenatal and postnatal steroid environment to sexual differentiation in the male and androgenized female using territorial marking behavior as an endpoint. Selective suppression of steroid effects in the pre- and postnatal period was accomplished with the antiandrogen flutamide or the antiestrogen MER-25. Treatment was given (1) prenatally (for 5 days before expected parturition), on the day of birth, and postnatally (to day 10) or (2) prenatally and on the day of birth only. Animals without postnatal antisteroid treatment were intact or were gonadectomized on day 2 and given testosterone propionate (TP) treatment on day 7. (It has previously been shown that day 7 is beyond the period of maximum steroid responsiveness in the male but not in the androgenized female.) MER-25, flutamide, or day-2 ovariectomy had no effect on adult marking behavior responsiveness in females given TP on day 7. All groups marked at normal male frequencies. The presence of flutamide prenatally and on the day of birth in day-2 castrates given TP on day 7 yielded adults with marking responsiveness equivalent to day-7 TP-treated females. In contrast, males given day-7 TP without prenatal and birthday flutamide showed significantly lower marking frequencies, suggesting that the presence of androgen prenatally and on the day of birth rendered day-2 castrates less responsive to TP given on day 7. The present study demonstrates that male-female differences in the neonatal period of maximum steroid responsiveness of the neural substrate which regulates marking behavior are not due to perinatal effects of ovarian steroids and can be attenuated or eliminated by inhibition of androgen effects in the male between day 5 prepartum and day 2 postpartum.