Studies of the reversible binding of [3H]cortisol by rat gastrocnemius muscle cytoplasm in vitro reveal specific binding in the 27,000 times g supernatant fraction at 0 degrees. The [3H]cortisol-binding molecule had an apparant Kd value of 1.7 times 10-7 M and the number of binding sites was 0.99 pmol per mg of cytosol protein. Only a single class of [3H]cortisol-binding sites could be detected, whose protein nature was suggested by its susceptibility to nagarse. The [3H]cortisol-protein complex sedimented at similar to 4 S in a 5 to 20% sucrose gradient either in the presence or absence of 0.3 M KCl. Binding increased more than 2-fold in adrenalectomized rats and was markedly reduced in the muscle of rats pretreated with cortisol. In contrast to the binding of [3H]dexamethasone and [3H]triamcinolone acetonide to receptor proteins in muscle, no correlation was found between the ability of various steroids to complete wtth [3H]cortisol binding and their glucocorticoid potency: [3H]cortisol binding was inhibited by a 1000-fold higher concentration of unlabeled cortisol and progesterone but not by dexamethasone or triamcinolone acetonide. It is therefore suggested that the [3H]cortisol-binding reaction is not directly involved in the biological effects of all potent glucocorticoids in skeletal muscle. The [3H]cortisol-binding protein in muscle cytosol could not be unequivocally distinguished from rat plasma corticosteroid-binding globulin, because both had similar steroid specificity and temperature stability, were not markedly affected by--SH reagents, and displayed similar sedimentation properties.