A method has been developed for measuring plasma testosterone in sheep using competitive protein binding (CPB) techniques. The procedure requires column chromatography on LH-20 Sephadex gel of a methylene chloride extract of plasma, and final determination of plasma testosterone by the CPB technique using salt precipitation of the protein bound fraction. The sensitivity of the assay has allowed application to the measurement of plasma testosterone levels in normal male, normal female, Klinefelter and freemartin sheep. It has also enabled the monitoring of the effect of stimulation - with intravenous pregnant mare serum gonadotrophin and human chorionic gonadotrophin (HCG) - on the plasma testosterone levels of normal male and Klinefelter sheep. A modification to the method, borohydride reduction of the initial methylene chloride extract, has enabled the plasma androstenedione levels to be determined, simultaneously with plasma testosterone, in two of the HCG stimulation studies.