Plasma fsh activity (as measured by the ovarian augmentation test in mice) was increased in gonadectomized male and female rats. A single injection of 20 μg oestradiol benzoate (OB) or one or two doses of 2·5 mg testosterone propionate (TP) 3 days later reduced the raised values significantly. Progesterone injected into spayed rats, primed 3 days before with OB or TP, induced, a few hours later, a significant increase in plasma fsh. By contrast, in male rats progesterone failed to exert a positive feed-back effect on the release of fsh. This sex difference was suppressed by the injection of testosterone into female rats on the 4th day of life. In male rats castrated soon after birth, progesterone showed a tendency to increase the activity of fsh in plasma in animals primed either with OB or TP, but the difference of the values as compared to the controls did not reach the level of significance. It is concluded that the sex difference of the positive feed-back effect of progesterone on the release of fsh could be due to a post-natal differentiation of a neural mechanism.