We compared the effects of 500 ml of 0.15M NaCl or 750 ml of 0.15M NaHCO3 infusions on PRA and PAC in 16 normal NaCl-restricted men. More positive sodium balance, greater natriuresis, and lower serum chloride, potassium, and hydrogen ion concentrations were observed after NaHCO3 infusion. PRA decreased (13.8 ± 1.0 ng/ml angiotensin I per hour to 6.6 ± 0.70) and PAC did not change (98 ± 15 ng/dl to 86 ± 15) after NaCl infusion. Conversely, PRA did not change (9.9 ± 1.6 to 10.1 ± 1.6) and PAC decreased (85 ± 9 to 44 ± 5) after NaHCO3 infusion. Plasma cortisol declined in both groups. These results suggest that in sodium-restricted man chloride is more important than sodium for the suppression of PRA by NaCl loading and that potassium or hydrogen ion is a more sensitive modulator of aldosterone secretion than is renin. © 1982.