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The effect of cortisol implants in the median eminence, midbrain reticular formation and ventral hippocampus has been assessed on basal adrenal activity. Two weeks after implantation rats were decapitated at 9-11 a m. or 4-5 p.m., and levels of adrenal and plasma corticosteroids were determined spectrofluorimetrically. Only implants in the median eminence produced a decrease in adrenal weight and a depression in a. m. adrenal corticosteroid levels. Implants in all three areas resulted in decreased levels of adrenal corticosteroids in the p. m. Implants in the ventral hippocampus and midbrain resulted in identical responses: plasma corticosteroids were increased in the a. m. and decreased in the p. m. Plasma corticosteroids were altered only in the p. m. following median eminence implants. The data indicate that the ventral hippocampus and midbrain are intimately involved in modulation of pituitary-adrenal function. It is suggested however, that cortisol implants in these areas may affect mechanisms in addition to those specifically related to pituitary-adrenal activity. [ABSTRACT FROM AUTHOR]