Earlier studies (Griffiths, 1963) have shown that the adrenal gland of the golden hamster possesses an active 19-hydroxylating enzyme system. This was demonstrated by measuring the conversion of 11-deoxycorticosterone (DOC) to 19-hydroxy DOC. The ability of this enzyme system to hydroxylate C19-steroids has now been investigated. Adrenal glands (300 mg.) from twelve male hamsters were homogenized in 500 μl. 0·25 m-sucrose containing 0·12 m-nicotinamide, as described earlier (Griffiths, 1963). The homogenate (450 μl.) was incubated with 3 μc [4-14C]testosterone (sp.ac. 8·8 μc/μmole) for 60 min. at 37° in 900 μl. medium containing 677 μmoles tris buffer, pH 7·4, 675 μmoles potassium chloride, 135 μmoles potassium fumarate, 2·94 μmoles NADP, 49·7 μmoles MgSO4, 14·7 μmoles ATP, 22·5 μmoles glucose-6-phosphate (G6P) and 1·0 Kornberg unit of G6P-dehydrogenase. Procedures for the isolation of the neutral steroid fraction, paper chromatography systems and steroid estimation and identification were those described