A method is described for the direct radioimmunoassay of aldosterone in human saliva without preliminary extraction or chromatographic purification. The method has been validated with respect to sensitivity, accuracy, precision, specificity and other criteria. Under basal conditions concentrations of aldosterone in saliva ranged from 10-63% those of plasma (mean +/- SD = 32.5 +/- 13.1%). When plasma aldosterone responded to such acute stimuli as postural change, exercise or ACTH infusion, this was promptly reflected in corresponding changes in saliva aldosterone concentration. In subjects going about their normal daily business saliva aldosterone concentration fluctuated markedly during the day. It is therefore difficult to define a normal range for saliva aldosterone concentration but present observations suggest an upper limit of 150 pmol/l under basal conditions.