In order to test if LH-RH also displays prolactin release-inhibiting activity, synthetic LH-RH was injected into intact diestrous rats and ovariectomized, estrogen-progesterone treated rats. Both groups of rats were pretreated with reserpine in order to increase serum prolactin levels. LH and prolactin were determined by radioimmunoassays. In spite of the fact that LH-RH brought about significant serum LH increase in both groups, no prolactin inhibition was seen. These results indicate that under the conditions of this investigation, LH-RH does not seem to display any prolactin release-inhibiting activity.