A single administration of morphine reduced both serum LH and FSH levels in castrated Wistar rats. As the inhibitory effect of morphine on serum LH was antagonized by naloxone, the opiate receptors are involved. Decrease in the serum FSH levels induced by morphine was considerably less than the decrease in serum LH, and naloxone did not significantly reverse this effect of morphine on serum FSH. Increase in the serum LH induced by exogenous LHRH was slightly potentiated and not inhibited by a large dose of morphine, in both intact and castrated rats. Increase in the serum FSH in intact male rats induced by exogenous LHRH was not altered by morphine. In the castrated animals, exogenous LHRH did not further increase the serum FSH levels and the latter was not altered by a concomitant administration of morphine. These results clearly show that the decrease in serum gonadotropin levels by morphine is not due to inhibition of the stimulatory effect of LHRH on the pituitary gland. The mechanism of the differential suppression on serum LH and FSH by morphine is postulated.