Previous work has demonstrated that several transition metals and their anions, including cadmium, arsenite, and selenite, can inhibit glucocorticoid binding to glucocorticoid receptors in vitro. In this study, we demonstrated that in vitro zinc can also inhibit the binding of glucocorticoids to their receptor at relatively modest concentrations (10 to 100 microM). This inhibition was demonstrated in both crude and immunopurified receptor preparations and was reversible following removal of zinc. Inhibition could also be reversed by addition of the reducing agent dithiothreitol (DTT). This suggested that zinc might be acting by interacting with the vicinal dithiols in the steroid binding region of the receptor as previously described for other transition metals and anions. The ability of a biologically important trace metal to block steroid binding suggests a role for zinc in the regulation of glucocorticoid receptor-ligand interactions and may explain the ability of zinc to block glucocorticoid-induced apoptosis.