In the first experiment ovariectomized female hamsters were administered varying dosages of progesterone (P), dihydrotestosterone (DHT) or CI-628 at the same time (concurrently) as estrogen (EB) or 48 hr after EB (sequentially). All groups also received 500 microgram P 4 hr before being tested for sexual receptivity. P was more effective in reducing receptivity when given sequentially with estrogen than when given concurrently. Thus, the inhibitory effect of P increased with an increased interval between EB and P treatment. More CI-628 than P was required to inhibit lordosis and unlike P, CI-628 was equally effective when given concurrently with EB as when given sequentially. DHT did not inhibit receptivity when given in either paradigm. In the second experiment ovariectomized hamsters were treated with varying dosages of DHT 12 hr before EB. An amount of DHT which had no effect in Experiment 1 significantly inhibited receptivity when given 12 hr before EB. The relative inhibitory effects of these three compounds were discussed in terms of the possible similarities and differences in their mechanisms of action for inhibiting lordosis.