Since the use of NH4CI – the standard agent for short duration acid loading – may be hazardous in patients with hepatic disease, the acute renal response to another acidifying agent, orally administered CaCl2 (2 mEq/kg body weight), was compared to that obtained with NH4CI (1.9 mEq/kg body weight) in normal subjects during a 5- to 6-hour test. Urine pH decreased in all subjects to less than 5.20, and there were no significant differences in the maximal renal responses to either agent. Both in terms of acidification of the blood and as a stimulus to lowering of urine pH and excretion of acid, CaCl2 can adequately replace NH4CI as an acidifying agent in a short duration test and can be used in clinical settings in which NH4CI is contraindica