1. The response to the short ammonium chloride acidification procedure in seventeen normal young men was carefully defined by performing this test with rigid control.
2. Wide variation in the response was observed, emphasizing the difficulty of interpretation of small changes when using this procedure.
3. After ingestion of ammonium chloride, urine pH correlated positively with both net acid and ammonium excretion contrary to present concepts. It is suggested that the renal capacity to produce ammonia dictates the relation of urine pH to net acid and ammonium excretion, respectively.