Clearance techniques were used to evaluate renal tubular sodium and water excretion in 4 patients with antibiotic-induced acute renal failure (ARF). Creatinine clearances and maximal urine flow rates of patients with ARF (22.6 and 5.23 ml/min, respectively) were significantly lower than control values during hypotonic volume expansion (125.5 and 13.71 ml/min, respectively, both p less than 0.01). During the period of maximal hydration, fractional sodium excretion (CNa/Ccr) and maximal urine osmolality (11.4% and 171 mosm/kg H2O, respectively) were increased compared to controls (1.04% and 53 mosm/kg H2O, respectively, both p less than 0.05). The increased CNa/Ccr observed in patients with ARF was consistent with reduced proximal sodium reabsorption as reflected by increased (CH2O + CNa)/Ccr and reduced fractional distal sodium reabsorption as indicated by decreased CH2O/(CH2O + CNa). The reduction in proximal and distal sodium reabsorption cannot be explained on the basis of an osmotic effect of urea as fractional clearances of BUN (CBUN/Ccr) were similar in patients with ARF and controls.