Total corticosteroid concentrations were determined in the plasma and brains of five species: mouse, rat, cat, monkey and man. Corticosteroid concentrations were measured also in the livers of mice, rats and monkeys. Competitive protein binding techniques were validated and used for the tissue corticosteroid assays. In each species the brain corticosteroid value was less than the total plasma corticosteroid level but greater than the presumed unbound fraction of plasma corticosteroids. A clear circadian variation of brain corticosteroid values was found in mice, together with a rapid elevation of the tissue levels in response to stress. Reduction of both plasma and tissue corticosteroid concentrations was observed after adrenalectomy and in response to dexamethasone treatment of mice. Between 24 and 48 h post morten, mouse brain corticosteroid values decreased greatly. In cat, monkey and human brains all regions examined contained approximately equal amounts of corticosteroids. In particular, brain areas which are not involved in pituitary-adrenocortical regulation contained large amounts of corticosteroids.