The aim of the present study was to evaluate the changes in the diurnal rhythm of the hypothalamic β-endorphin (β-EP) contents in female rats as a function of circulating estrogens. With this purpose we evaluated the diurnal hypothalamic β-EP changes (1) during the estrous cycle, and (2) in ovariectomized rats with and without acute and chronic estrogen replacement. Ovariectomized rats were treated either acutely with 10 µg of estradiol benzoate (EB) or chronically with 2 µg/day of EB for 15 days. β-EP concentrations were measured in acid extracts of medial basal hypothalamus by a specific radioimmunoassay. During the estrous cycle, hypothalamic β-EP concentrations showed a significant nocturnal increase, with no difference between the 4 days of the cycle. On the day of estrus, β-EP concentrations between 12.00 and 18.00 h resulted significantly lower than in the other days of the cycle. After ovariectomy, the night-related changes in hypothalamic β-EP disappeared. The acute administration of EB induced a significant increase in hypothalamic β-EP after 21 h (18.00 h). On the other hand, the chronic replacement restored the nocturnal peak of hypothalamic β-EP (18.00, 21.00, 24.00 h). The present data emphasize the role of central β-EP in regulating the reproductive functions. Moreover, the effect of estrogen in modulating the circadian changes in hypothalamic β-EP supports the important role of estrogens in br