The effects of famotidine on renal function were investigated. Eight healthy men (N) and 8 renal patients with varying degrees of renal failure (R.F.) participated in the trial. They were given either famotidine (40 mg) or cimetidine (800 mg) for 7 days. Cimetidine produced a significant decrease in creatinine clearance (from 131.6 +/- 12.9 to 107.7 +/- 3.4 ml/min (N), and 22.2 +/- 3.9 to 18.1 +/- 3.5 ml/min (R.F.], and increase in serum creatinine (from 0.96 +/- 0.05 to 1.09 +/- 0.04 mg/dl (N), and 3.46 +/- 0.62 to 3.86 +/- 0.51 mg/dl (R.F.], and a decrease in creatinine excretion (from 24.0 +/- 1.1 to 22.6 +/- 0.8 mg/kg/24 hr (N], respectively, although the reduction in creatinine excretion in the renal failure group was small. On the other hand, the famotidine treatment produced no significant changes in renal function in both subjects (creatinine clearance, 136.5 +/- 5.7 to 133.6 +/- 6.0 ml/min (N), and 21.9 +/- 3.6 to 20.9 +/- 4.1 ml/min (R.F.); serum creatinine, 0.98 +/- 0.02 to 0.99 +/- 0.02 mg/dl (N), and 3.24 +/- 0.43 to 3.46 +/- 0.55 mg/dl (R.F.); and urinary creatine, 25.1 +/- 1.0 to 25.3 +/- 1.0 mg/kg/24 hr (N), and 17.2 +/- 1.4 to 16.6 +/- 1.5 mg/kg/24 hr (R.F.]. There were no changes in the percent sodium excretion, or the serum and urinary beta 2-microglobulin in both groups.