Summary. The pattern of plasma LH levels in castrated rams after injection of testosterone propionate (TP) was biphasic with significant decreases at 12 and 72 hr after injection. The total LH response of similarly treated rams injected with LH-RF at critical times in the biphasic LH pattern was of three types regardless of the dose of LH-RF: (i) a similar response to that of control animals at 12, 72 and 120 hr after TP, (ii) a greater response than that of control animals at 36 hr after TP (this is regarded as a rebound effect after a partial inhibition of the release), and (iii) a lower response than that of control animals 6 hr after TP. The third type of response suggests that in the ram testosterone may act directly at the pituitary level to decrease the responsiveness to LH-RF.