The present study was designed to test the effect of acute administration of dexamethasone on the postcastration and gonadotrophin-releasing hormone (GnRH)-induced rise of LH, and to examine whether the inhibitory action of glucocorticoids on LH secretion was mediated by the opioid system. Rats were castrated and injected 10-10.5 h later in a first set of experiments with saline, dexamethasone (250 micrograms/rat), nalmefene (2 mg/kg body weight) or nalmefene plus dexamethasone. The response to GnRH was tested 11 h after castration (time 0) in all groups. Dexamethasone caused a significant (P less than 0.005) decrease in basal serum concentrations of LH, while saline and nalmefene did not induce any change. The administration of dexamethasone preceded by nalmefene increased basal concentrations of LH (1.6 times), with an effect significantly greater than that of dexamethasone (P less than 0.001), nalmefene (P less than 0.05) or saline (P less than 0.005) alone. GnRH induced a significant (P less than 0.001) increase in serum concentrations of LH in all groups. In a second set of experiments, administration of naloxone (2 mg/kg body weight) increased LH levels (P less than 0.05) and similarly reversed the inhibitory effect of dexamethasone on LH.(ABSTRACT TRUNCATED AT 250 WORDS)