Chronic metabolic acidosis (CMA) in human beings is characterized by increased renin-angiotensin-aldosterone (RAA) activity and cortisol secretion as well as nitrogen wasting. The purpose of this study was to examine whether and to what extent increased RAA activity (i.e., angiotensin II or aldosterone) regulates acid-base equilibrium in CMA and thus might co-determine the severity of acidosis. CMA was induced in 8 normal subjects by oral NH4Cl administration (2.1 mmol/kg body weight per day) for 7 days, followed by a 7-day period of spironolactone (100 mg, 4 times a day by mouth), followed by a 4-day period of spironolactone and losartan (100 mg, every day by mouth). NH4Cl feeding was continued during all study periods. Spironolactone resulted in exacerbation of acidosis ((HCO3)p decreased from 19.8+/-0.4 mmol/L to 17.7+/-0.6 mmol/L, P<.005) because of a large increase in endogenous acid production, as evidenced by significant increases in net acid excretion (116 to 185 mmol/day, P<.005), urinary anion gap (+31 mEq/day, P<.05), and sulfate excretion (+32 mEq/day, P<.05). Plasma potassium increased from 4.2 to 4.6 mmol/L (P<.05) because of decreased urinary potassium excretion (from 108 to 92 mmol/day, P<.05). Plasma angiotensin II, cortisol, aldosterone, urinary aldosterone, urinary tetrahydrocortisol, free cortisol, and nitrogen excretion increased significantly. The subsequent addition of losartan to spironolactone administration resulted in further exacerbation of acidosis ((HCO3)p decreased to 15.7+/-0.4 mmol/L, P<.05) and hyperkalemia (5.0 mmol/L, P<.05) with no change in plasma anion gap. Renal potassium excretion decreased from 92 to 73 mmol/day (P<.05) on day 1. Exacerbation of acidosis was accounted for by a renal mechanism, as evidenced by the significant decrease in net acid excretion and unchanged urinary unmeasured anion and nitrogen excretion. We conclude the following: (1) AT-1 blockade by losartan exacerbates acidosis by inducing a distal-tubular acidification defect. Angiotensin II is an important modulator of the renal acid excretory response to CMA in human beings. (2) Inhibition of aldosterone action by spironolactone in CMA results in an increase in endogenous acid production and exacerbates acidosis by a non-renal mechanism that is mediated, at least in part, by exacerbated hyperglucocorticoidism.