Urinary albumin and beta 2-microglobulin excretion and creatinine clearance were measured during 8 weeks' conventional cimetidine treatment in 13 ulcer patients without signs of renal disease. No changes were found in albumin or in beta 2-microglobulin excretion, indicating no changes in glomerular permeability and tubular reabsorption of proteins in the kidney. Creatinine clearance fell about 28 ml/min (26%) during cimetidine treatment but normalized after termination of medication. Concomitantly, serum creatinine increased by 22% during treatment and fell again after termination of treatment. The mechanism behind the changes in renal handling of creatinine during cimetidine was not clarified in the present study. It is concluded that the conventional 8 weeks' cimetidine treatment seems safe from the renal point of view, at least in patients without signs of renal disease.