Diurnal variations of the effectivity of β-endorphin (β-End), dynorphin (DYN), Met-enkephalin (Met-Enk), D-Met2-Pro5-enkephalinamide (D-Met-Pro-Enk) and morphine to induce prolactin (PRL) and adrenocorticotropin (ACTH)&slash;corticosterone (CS) release in intact and adrenalectomized rats have been examined. The response to morphine (10 mg/kg s.c), Met-Enk (200 µg/rat i.c.v.) and D-Met-Pro-Enk (0.5 µg/rat i.c.v.) did not change with different times of the day, while that to β-End (0.5 µg/rat i.c.v.), DYN (1 µg/rat i.c.v.) and U50–488H, a selective kappa agonist (10 mg/kg s.c), showed a circadian rhythm in stimulating PRL release, with a higher increase in the afternoon (16.00–17.00 h) than in the morning (08.00–09.00 h). In adrenalectomized rats the loss of this circadian rhythm was shown. The CS release evoked by morphine, D-Met-Pro-Enk, Met-Enk and DYN was demonstrable only in the morning when the basal CS level was significantly lower than in the afternoon. The afternoon release of ACTH by morphine was higher than in the morning in adrenalectomized rats. β-End and U50–488H were equally active in the morning and in the afternoon in increasing CS secretion. The present results suggest that the diurnal rhythm in the response of CS and PRLrelease to opioids is in relation with the glucocort