Previous in vitro binding studies with androgen receptors in rat seminal vesicles (Toth M. and Zakar T., J. steroid Biochem. 17 (1982) 653-660) have shown that the difference in the effects of nandrolone (N) and testosterone (T) is caused by the fact that 5 alpha-reduction increases the affinity of T and decreases the affinity of N. We confirmed this result using androgen receptors in rat prostate and intact human MCF-7 cells. We also analysed the receptor binding properties of N, T, dihydronandrolone (DHN) and dihydrotestosterone (DHT) in vivo following a combined 2-h infusion of a physiological dose of [3H]T and the same dose of [3H]N in castrated male rats, which permitted a direct comparison of the accumulation of [3H]N, [3H]T, [3H]DHN and [3H]DHT in different sub-cellular fractions of various tissues. There was a considerable accumulation of radioactivity in the liver, but no retention of active compounds. In the prostate there was a preferential retention of [3H]DHT over [3H] DHN in the receptor fractions whereas in thymus, spleen and muscular tissues [3H]N and [3H]T were retained in equal amounts. The kidney showed a preferential retention of [3H]N over [3H]T. The present results explain the relatively strong effect of nandrolone compared to that of testosterone on target tissues devoid of 5 alpha-reductase activity (e.g. muscular tissue) compared to its relatively weak effect on tissues with a relatively high 5 alpha-reductase content (e.g. prostate).