Serum creatinine, endogenous creatinine clearance, (51Cr)EDTA plasma clearance and the concentration of beta2-microglobulin in serum and urine were determined in 19 patients before and during treatment with cimetidine for peptic ulcer disease. The mean concentrations of creatinine and beta2-microglobulin in serum increased significantly within normal limits after 2 and 6 weeks' treatment. However, creatinine clearance and (51Cr)EDTA plasma clearance were unchanged during the treatment. Thus, the observed increases in serum creatinine and beta2-microglobulin could not be explained by a diminished glomerular filtration rate. Inhibited tubular secretion of creatinine may explain the observed increase in serum creatinine during cimetidine treatment. Another hypothetical possibility is that a small increase in tubular reabsorption of both creatinine and beta2-microglobulin would account for the observed increases in creatinine and beta2-microglobulin in serum. It is concluded that, although statistically significant, the increases in serum creatinine and beta2-microglobulin are small and therefore of little relevance for the patient's treatment with cimetidine.