Plasma aldosterone concentration (PAC) response to ACTH is utilized in clinical and experimental protocols. However, PAC response to ACTH in normal subjects controlled for modifiers of PAC has not been established. Our report includes two experiments. In both, subjects were studied in the morning and were in sodium (Na+) balance prior to study on 4 g Na+ for phase I, and 2 and 8 g Na+ diets for phase II. Na+ balance was established by 24-h urinary Na+ (UNa+) in phases I and II, and also by fractional excretion of Na+ (FENa+) in phase II. After being supine for 60 min, subjects received 0.25 mg of iv bolus ACTH. PAC, plasma renin activity and plasma potassium (K+) were drawn every 30 min in phase I and every 15 min in phase II. The rise in PAC (rPAC) and correlation coefficients were calculated. In phase I, peak PAC occurred at 30 min, 1130 +/- 420 pmol/L, with a rPAC of 810 +/- 310 pmol/L. Twenty-four hour UNa+ was 86 +/- 45 mmol/24 h. In phase II, time and magnitude of peak PAC and rPAC were dependent on diet. Both occurred at 30 min for 8 g Na+: peak PAC was 640 +/- 210 pmol/L and rPAC was 440 +/- 190 pmol/L; and at 60 min for 2 g Na+: peak PAC was 1040 +/- 320 pmol/L and rPAC was 690 +/- 220 pmol/L. Correlation coefficients for rPAC and 24-h UNa+ was r = -0.44, P less than 0.05 and for rPAC and FENa+ was r = -0.46, P less than 0.05. In summary, in subjects supine for 60 min prior to iv bolus ACTH, Na+ balance is the most important determinant of PAC response. Both magnitude and timing of rPAC is influenced by Na+ balance. Finally, both 24-h UNa+ and FENa+ are valid for establishing pretesting Na+ status.