Abstract 1. (1) A simplified fluorimetric procedure for the estimation of free cortisol and corticosterone in 1-ml samples of human plasma has been described. 2. (2) A correction procedure for non-specific fluorescence has been described which is applicable to both human and rat plasma. It depends on the differing rates of development of fluorescence of plasma and of solutions of cortisol and corticosterone. 3. (3) The mean value for free plasma cortisol in 27 normal adults was 9.6 ± 2.7, and for free plasma corticosterone 0.4 ± 0.2 μg/100 ml. Elevated values were foundin Cushing's syndrome, and normal levels in hirsutism and in Addison's disease. Low values occurred in hypopituitarism.