To evaluate the effect of acute histamine H2-receptor blockade on renal function, renal function studies were performed in a control state and after cimetidine. Studies included acid excretion in response to acid loading, bicarbonate reabsorption during bicarbonate infusion, and urinary concentrating ability. Cimetidine produced no significant effect on any of these functions. During bicarbonate infusion, inulin clearance remained constant while creatinine clearance fell, possibly because of an effect on tubular creatinine secretion.