The effect of intravenous diazepam on glomerular filtration rate (inulin clearance) and effective renal plasma flow (PAH clearance) was investigated in 6 children and 12 anesthetized rabbits. A transient decrease in inulin and PAH clearances was observed in 6 children given 4 mg of diazepam intravenously, without measurable change in blood pressure. A similar renal effect was observed in anesthetized rabbits, together with a transient drop in systemic arterial pressure. Continuous infusion of diazepam (5 mg/kg/hr) did not affect renal function in rabbits. We suggest that this effect of diazepam should be borne in mind when the drug is administered to patients undergoing renal clearance studies or to patients with preexistent renal insufficiency.