To investigate the mechanism of androgen-induced thymolysis, the effects of various androgens, including testosterone (T), 19-nortestosterone, and 7 alpha-methyl-19-nortestosterone (MENT), were compared with those of estradiol and dexamethasone (DEX) in intact, castrated, and adrenalectomized male rats. The potency comparisons on thymus regression, based on mass of steroids, showed DEX to be the most potent, followed by estradiol and the androgens. Among the androgens, MENT was the most potent, followed by nortestosterone and T, an order similar to their anabolic potency on muscle. As the thymolytic effects of T and MENT were not altered by the concomitant administration of an aromatase inhibitor or a 5-reductase inhibitor, it was concluded that the effects of androgens were not mediated by their conversion to estrogens or 5 alpha-reduced metabolites. Involvement of glucocorticoid receptors in androgen action was excluded because mifepristone (an antiglucocorticoid) blocked DEX-induced, but not T- or MENT-induced, thymus regression. Flutamide, an antiandrogen, significantly blocked the thymolytic effect of T and MENT, providing further support for this conclusion. This suggested that the thymolytic action of androgens is an intrinsic property mediated via androgen receptors (AR). The occurrence of AR in the thymus was demonstrated by binding assays and the presence of AR messenger RNA (mRNA) by reverse transcriptase-polymerase chain reaction. Quantitative reverse transcriptase-polymerase chain reaction for AR mRNA in the thymus showed 6-fold more AR mRNA in the thymic epithelial cells than in the thymocytes. However, epithelial cells represent only a small fraction of the thymus. Hence, it is hypothesized that the androgens produce their thymolytic effects by stimulating the secretion of a factor(s) by the thymic epithelial cells that, in turn, causes regression of the thymus.