Young (3-week-old) spontaneously hypertensive rats (SHR) had significantly higher basal plasma corticosterone levels than WKY rats and maximum responses to ACTH were also higher. In isolated adrenocortical cells from these rats, corticosterone production was also more responsive to ACTH in SHR. There was no significant difference in aldosterone production. Mononuclear leucocytes from older (10-week-old) SHR had a higher affinity for dexamethasone but a smaller number of binding sites per cell. The SHR therefore has higher circulating glucocorticoid levels and the target cells have a higher apparent affinity for this agonist. However, the target cells also have a smaller binding capacity. The precise resultant effect of these changes on glucocorticoid activity will require additional studies on specific glucocorticoid-dependent variables.