The purpose of this study was to determine the duration of the hormone-sensitive postnatal period during which a single injection of testosterone propionate (TP) influences feminine behavior and luteinizing hormone (LH) secretion in male and female rats. Male pups were castrated on the day of birth (day 1) between 6 and 12 h postpartum. On postnatal day 3, 4, 5, 6, 7, 8, or 9 female pups and castrated males (fales) were injected subcutaneously with testosterone propionate (TP-500 micrograms). The females were laparotomized at 60 days of age, and ovarian tissue was removed for histological analysis. Female behavior was evaluated at 100 days of age. At 150 days of age, the ability of steroids to facilitate LH secretion was determined. Tests for lordosis indicated a diminished lordotic quotient (LQ) with both females and fales treated with TP on postnatal day 3, 4, 5, 6, or 7. On day 8 or 9, however, the lordotic response was at control levels. Females in all TP treated groups had significantly reduced number of corpora lutea. Females and fales treated with TP on postnatal day 3, 4, 5, 6, 7, 8, or 9 failed to exhibit an LH surge as adults. The results indicate that the neural control of feminine behavior (LQ) is hormone-sensitive to a single injection of TP up through the 7th day of postnatal life, whereas the neural substrate regulating LH secretion was sensitive in the present study at least up through day 9 in both fales and females.