The role of the pituitary-adrenal axis on receptor binding and diurnal rhythmicity of benzodiazepines (BNZ) was assessed in the rat cerebral cortex. Groups of intact, adrenalectomized (ADx) and/or hypophysectomized (HPx) rats were killed at six different time intervals during the 24-hour cycle. BNZ binding was estimated by Scatchard analysis of <sup>3</sup>H-flunitrazepam high-affinity binding to rat cerebral cortex. Intact and sham ADx animals show a similar pattern in diurnal thythm of BNZ binding, with a maximal concentration at midnight. Bilateral ADx induced a significant increase in B<sub>max</sub> at all time intervals studied, the largest rise appearing at midnight. HPx alone led to a slightly smaller rise in B<sub>max</sub> than in ADx rats, while HPx performed in ADx rats did not modify the response to ADx alone. B<sub>ma</sub>χ of BNZ binding in ADx rats reached maximal values at 3–7 days after surgery, and decreased somewhat at 15 days post-ADx. Corticosterone administration at a single dose of 5 mg i.p. 24 h before sacrifice returned B<sub>max</sub> to normal values in ADx as well as in ADx plus HPx rats. The corticosterone effect is not exerted on the BNZ binding sites themselves, as revealed by the lack of effect of this glucocorticoid in vitro. These findings indicate that BNZ receptors in rat cerebral cortex can be modified by the adrenal gland, with corticosterone as a primary effector.