Norfloxacin is an antibacterial drug chiefly eliminated by the kidney and therefore useful in the treatment of urinary tract infections. To study its pharmacokinetics in chronic renal failure, we administered a single oral dose of 400 mg to 14 patients and 6 controls with normal renal function. Patients were divided into three groups according to the severity of renal failure. Norfloxacin was measured in serum and urine by bioassay. Serum half-life in controls was 3.87 hours and prolonged to 5.85 hours in group I (creatinine clearance 80-45 ml/min), 7.25 hours (p less than 0.05) in group II (creatinine clearance 44-20 ml/min) and 8.34 hours (p less than 0.01) in group III patients (creatinine clearance less than 20 ml/min). A good linear correlation between the elimination rate constant and creatinine clearance (r = 0.79, p less than 0.01) has been found. Total urinary excretion in the 72 hour period after administration achieves 40.4% in controls, falls to 23.5% (p less than 0.05), 15.6% (p less than 0.01) and 8.2% (p less than 0.01) of administered dose in groups I, II and III, respectively. Similarly, urinary concentrations decrease in all patient groups with respect to controls. Our data show that effective urinary antibacterial concentrations of norfloxacin after 400 mg single oral dose were obtained even in patients with severe renal failure. In these patients systemic accumulation after repeated dose administration is a probable event. Therefore, dosage adjustment is advisable in patients with creatinine clearance less than 20 ml/min, although it will inevitably cause lower urinary concentrations.