The purpose of this study was to investigate the effect of oral administration of progesterone (15 micrograms norethindrone, NE) in presence and absence of estradiol (1 microgram ethinyl estradiol, EE2) on the CNS levels of beta-endorphin like immunoreactivity (beta-EI) in female rats. In acute study (5 days), NE alone did not change beta-EI significantly in pituitary. NE and EE2 together decreased beta-EI by 37% (47% at 10X dose). In chronic study (7 weeks), 2NE had no significant effect on pituitary beta-EI, however, NE and EE2 together at 10X dose decreased it by 14%. In the hypothalamus, NE alone or in presence of EE2 had no significant effect on beta-EI, but 10X dose of NE+ EE2 caused 50 and 76% decrease in beta-EI in acute and chronic study. Striatum was the only tissue where NE alone caused a decrease of 82% in beta-EI when given acutely and 52% when given chronically. EE2 had some protective effect on this decrease since when given together (NE+EE2) the decrease in beta-EI was 21% in acute and 43% in chronic study. Thus our results, along with other studies on the regulation of gonadotropin levels by opioids, suggest that oral contraceptives alter the level of beta-EI and in turn may regulate the release of gonadotropins. Morphine and endogenous opioids have been shown to decrease gonadotropin secretion in various species including humans, apparently by suppressing the release of LH-RH from the hypothalamus (1-5). The opiate antagonist naloxone not only causes up to 10-fold increase in the secretion of gonadotropins (1,3, 6-9) but also opposes the negative feedback effect of steroids on the hypothalamic-pituitary-gonadotropin axis (8), suggesting a regulatory interaction between the endogenous opioids, gonadotropins and gonadal steroids. Like ACTH, the secretion of beta-endorphin is inhibited by glucocorticoids (10). Naloxone induced release of LH is facilitated by estradiol in humans (11) suggesting an antagonistic effect of estradiol on the endogenous opioids. beta-endorphin may play a significant role in neurochemical mechanisms of gonadotropin release in the human menstrual cycle. A preovulatory increase of beta-endorphin in serum occurs 2 days prior to LH surge and a postovulatory decrease in beta-endorphin occurs 5 days later.(ABSTRACT TRUNCATED AT 400 WORDS)