Normal rat-thymus cytosol was shown to contain a 5 alpha-dihydrotestosterone-binding species characterised by low capacity and high affinity (Kd-6 X 10(-10) M). The principal binding moiety had a sedimentation coefficient of 8 S at low ionic strength. Competitive binding studies with 61 compounds showed that the specificity was highly androgenic while comparison with similar data from the prostate indicated that the thymic androgen receptor had a different specificity pattern. The thymus was not found to be capable of 5 alpha-reduction; unlike the prostate, it rapidly metabolised testosterone to androstenedione. Androgen receptor levels were significantly higher in intact females than males and the binding species was positively identified only in medullary tissue--thymocytes were devoid of the receptor. This data supports the idea that the thymus is an androgen-responsive tissue and that at least part of the immunoregulatory effects of androgens are mediated through the thymus.