The effects of the administration of naloxone hydrochloride (NX) or WIN 44,441-3 (WIN), administered either alone or prior to the subsequent administration of the three prototypic multiple opiate receptor agonists morphine sulfate (MS), ethylketocyclazocine methanesulfonate (EKC) and N-allylnormetazocine hydrochloride (NANMT), on the release of anterior pituitary hormones were studied in the rat. Serum levels of corticosterone, growth hormone, prolactin and luteinizing hormone were measured by radioimmunoassay. The administration of WIN alone produced a pattern of hormone release similar to that seen following the administration of NX. Pretreatment with either NX or WIN blocked the MS-induced rise in serum levels of corticosterone whereas neither the EKC nor the NANMT-induced rises were blocked. Pretreatment with WIN blocked the EKC-induced rise in serum growth hormone but failed to block the MS-induced increase. Both antagonists blocked the rise in serum levels of prolactin induced by either MS or EKC but also blocked the NANMT-induced fall. The administration of either NX or WIN blocked the inhibition of luteinizing hormone release induced by either MS or NANMT and partially blocked the EKC-induced fall. The data indicate that multiple opiate receptors are involved in opiate-induced changes in anterior pituitary hormone release and suggest that the patterns of hormone release induced by various opiate agonists as well as their interaction with antagonists may be useful in classifying drugs with respect to their activity toward specific receptors.