Resting levels of plasma and adrenal corticosteroids, pituitary content of adrenocorticotropin, and circulating leukocytes were determined at intervals during controlled 24-hr light-dark cycles in intact, castrated, sham-castrated adult and prepubertal male and female rats. To study the influence of environmental lighting rhythms, corticosteroid levels were similarly followed in intact and blinded male and female rats and in ovariectomized females following a 9-hr shift in lighting regimen. All groups of animals showed evidence of cyclic pituitary-adrenal function, but the presence of mature ovaries was associated with marked facilitation of the diurnal excursions in corticosteroid levels. Furthermore, the results indicated that the mechanisms responsible for pituitary-adrenal rhythmicity are influenced by cyclic ovarian function, are sensitive to pentobarbital, and are synchronized by environmental lighting rhythms perceived through the eyes. Several of the features of pituitary-adrenal function under resting conditions resemble those associated with cyclic release of gonadotropin leading to ovulation. Similar or overlapping neural mechanisms may be responsible for these endocrine rhythms.