Neurotensin (NT), substance P (SP) and morphine sulfate (MS) elevate plasma prolactin and growth hormone levels in both normal or estrogen-progesterone pretreated male rats. By contrast, steroid priming is required for TRF to exhibit PRL-releasing activity. Naloxone, an opiate receptor blocker, reverses the stimulatory effect of MS only. Diphenhydramine, a histamine antagonist, inhibits the response to NT, SP and MS without affecting the response to TRF. These results suggest the involvement of a histaminic step in the action of NT, SP and MS. TRF, NT and SP do not appear to stimulate PRL and GH through activation of an opiate receptor.