Serum creatinine and beta 2-microglobulin levels, endogenous creatinine clearance, the urinary beta 2-microglobulin excretion rate, and urinary protein levels were monitored serially in 29 patients with duodenal ulcers randomly allocated to receive eight weeks of treatment with cimetidine, ranitidine, or an antacid. A clinically insignificant increase in endogenous creatinine clearance was noted in patients given ranitidine. No adverse change in any of the variables measured was detected in any group of patients during the 12-week period of study.