We examined the application of a radioimmunoassay kit, not requiring chromatography, to the measurement of aldosterone in healthy human subjects. Plasma aldosterone, urinary aldosterone, and plasma renin activity measurements were performed on samples from 47 subjects under conditions in which dietary sodium intake, posture, and times of sample collection were rigidly controlled. Nomograms are presented to illustrate the continuous, rather well defined, inverse relationship between 24 h sodium excretion and aldosterone measurements in plasma and urine. Mean plasma and urinary aldosterone values for subjects on normal sodium diets were 0.332+/-0.211 nmol/liter and 36.6+/-18.6 nmol/24 h and agreed well with previously reported values. A significant (p less than 0.05 increase in 24 h urinary aldosterone, plasma aldosterone and plasma renin activity resulted with decreased dietary sodium intake and conversely, a significant (p less than 0.05) decrease in each hormone occurred with increased dietary sodium intake. Rising from the supine to upright position resulted in a 230% increase in plasma aldosterone concentrations and a 284% increase in plasma renin activity.