In an attempt to evaluate the effect of LRH on GH and PRL secretion, LRH in a dose of 100 micrograms was injected iv to 22 patients with active acromegaly. A definite increase in both serum GH and PRL levels was observed in one patient, while four other subjects responded to LRH to secret either GH or PRL. The effectiveness of LRH on GH and PRL release was not correlated with the age or sex of patients, the basal levels of serum GH or PRL, the GH response to TRH administration, or the increase in serum gonadotropin concentrations after LRH injection. In three patients, this LRH-induced GH and PRL release was completely blocked by pretreatment with 2-bromo-alpha-ergocriptine (CB154). The results suggest that elevations of GH and PRL after LRH occur rather sporadically in acromegalic patients and that the action of LRH on GH release is occasioned by a different mechanism from that of TRH.