The occurrence of steroid 5α-reductase activity has been investigated in nuclear preparations of various organs from rats. The kinetic properties of the enzyme have been studied in renal nuclei in view of the apparent involvement of the 5α-reductase in androgen action. Special attention has been paid to the substrate specificity of the enzyme and the influence of natural and synthetic androgen antagonists upon its activity. It has been concluded that the occurrence of the enzyme in nuclear preparations is not limited to male target organs and that the activity of this 5α-reductase is not limited to androgens. Preliminary data concerning the cell distribution of the enzyme indicate that about 16% of the enzyme activity is associated with purified nuclei in the rat kidney. In the rat prostate the nuclear activity reaches 28 % of the total cellular activity.