When tested in an in vitro assay system, progesterone and various analogs of this steroid were shown to compete with [3H] triamcinolone acetonide (TA) for specific glucocorticoid receptors in both rat liver and thymus. Of these analogs, the following derivatives of progesterone were potent competitors of TA binding and, when injected into adrenalectomized rats, induced regression of the thymus and marked increases in hepatic tyrosine aminotransferase activity: 11 beta-hydroxyl, 6 alpha-methyl, 6 alpha, 16 alpha-dimethyl, and 6 alpha-methyl-17 alpha-hydroxyl. In contrast, progesterone, 16 alpha-methyl, and 17 alpha-hydroxy progesterone competed with TA in vitro but failed to elicit either gluco- or antiglucocorticoid activity in vivo. Also, we observed that the oral contraceptive 6 alpha-methyl-17-(1-propynyl)testosterone competes very effectively with TA in a cell-free preparation of rat liver and induces an increase in hepatic tyrosine aminotransferase activity. The 11 beta-hydroxyl group has previously been thought to be essential for glucocorticoid activity. Our studies indicate that substitution of progesterone or testosterone with a 6 alpha-methyl group negates the need for an 11 beta-hydroxyl substitutuent as a prerequisite for glucocorticoid activity.