The microsomal fraction (10 000–105 000 × g precipitate) of equine testes was fractionated into the smooth- and the rough-surfaced microsomal subfractions by a sucrose density-gradient centrifugation in the presence of CsCl. The validity of this fractionating procedure was confirmed by electron microscopic examination and also by chemical analysis of the RNA contents in these subfractions. The aromatizing enzyme system (19-hydroxylase and aromatase) which was concentrated in the microsomal fractions among the organellae was found to be localized in the smoothsurfaced microsomal fraction. The cytochrome P-450 which was also involved in the process of enzymatic aromatization was detected exclusively in the smooth-surfaced microsomal fraction. The distribution of the aromatizing system between the two microsomal subfractions of equine testes was discussed in comparison with that in human full term placentae.