The inhibitory effect of progesterone (P) injected at various times on female sexual behavior was investigated in estradiol benzoate (EB) treated ovariectomized rats. Four behavioral tests were carried out at two-week intervals. All females received 5 μg/kg b.w. EB and 0.5 mg P 44 hr after the EB. In the P-control group, an additional 5 mg P was administered at the same time as the injection of EB in four tests. Instead of P, oil was given concurrently with EB in the Oil control group. In the experimental groups, female rats were treated with 5 mg P from 1 to 40 hr before (PB group) or after (PA group) the EB-injection. A sexual behavioral test was started 4 hr after 0.5 mg P. The results show that low levels of lordosis and soliciting behavior were observed in the P group, compared to the Oil-control group. In the PB groups, lordosis quotient (LQ) was low when P was given from 1 to 24 hr before EB. Moreover, animals in which P was given 27-40 hr before EB showed lower LQ than Oil-control animals, but higher LQ than rats in the P-control group. In the PA groups, when P was administered from 1 to 24 hr after EB, low levels of lordosis response were observed, whereas animals which received P 27-40 hr after EB showed'a high score of LQ, being comparable to that in the Oil control. These results suggest that the period of 24 hr before and after EB injection is a critical period for inhibitory action of P on female sexual behavior in female rats.