In this investigation an attempt was made to find a compound based on urinary steroids which characterise the mean cortisol metabolites. Free urinary cortisol turned out to be useful for this purpose. Thus a positive correlation can be demonstrated under basal conditions as well as after standardized loading tests with metyrapone and β1-24-tetracosactide2). With regard to the lens-effect described in a previous paper and the mechanisms leading to this effect it was considered whether the determination of free urinary cortisol could be used as a sensitive estimation of enhanced adrenal activity, providing this rise in activity led to an increase of the biologically active plasma cortisol fraction. Further studies of free urinary cortisol in pathological conditions other than Cushing's syndrome as well as calculations of ratios of urinary C-17-OHCS and plasma cortisol are necessary, however, in order to evaluate the diagnostic possibilities in use of free urinary cortisol as a parameter.