The effects of intraventricular injection of beta-endorphin on initial estrogen action to induce lordosis behavior. Torii M,Kubo K Physiology & behavior Ovariectomized female rats subcutaneously (SC) injected or intracerebrally implanted with estradiol benzoate (EB), and given progesterone SC were used as experimental animals to assess the effects of the beta-endorphin (beta-EP) neuronal system on lordosis behavior. In intraventricular (IV) injection of beta-EP at the onset of sc EB priming, the lordosis behavior was significantly (p < 0.001) facilitated. In contrast, the lordosis behavior was significantly (p < 0.001) inhibited by IV injection of naloxone, an opioid receptor antagonist. beta-EP facilitation of lordosis was observed exclusively within the initial stage of estrogen action. The behavior was significantly (p < 0.001) facilitated by IV injection of beta-EP given with an intracerebral implantation of crystalline EB into the septal-preoptic regions. However, the lordosis behavior was significantly (p < 0.001) inhibited by beta-EP when EB was implanted into the ventromedial hypothalamus. Animals receiving EB implants into the mesencephalic reticular formation were not affected by beta-EP. The present study suggests that the beta-EP neuronal system stimulates sexual receptivity through an action on the central nervous system in relation to the site of estrogen-initial activation to induce the lordosis reflex. The sites of beta-EP action may be the estrogen receptive septal-preoptic and hypothalamic regions; the former for facilitatory effect and the latter for inhibitory effect. 10.1016/0031-9384(94)90024-8