SUMMARY An assay was devised, using Sephadex LH-20 chromatography and competitive protein binding, to measure cortisol, corticosterone, 11-deoxycortisol and cortisone in a single extract of 4 ml peripheral plasma or 50–300 μl adrenal venous plasma in sheep. The method is especially suited for measuring the low concentrations of these steroids in ovine plasma. Mean values (μg/100 ml) in dexamethasone-suppressed plasma were cortisol, 0·042; corticosterone, 0·004; 11-deoxycortisol, 0·004; and cortisone, 0·021. Mean values (μg/100 ml) in basal plasma were cortisol, 1·11; corticosterone, 0·044; 11-deoxycortisol, 0·020; and cortisone, 0·24. Peripheral concentrations of all four steroids increased in response to corticotrophin, but none increased after angiotensin. Metyrapone administration produced a marked rise in deoxycortisol. The method provided direct evidence that cortisone and deoxycortisol were secreted by the adrenal gland in sheep.