1. Sodium excretion, plasma renin activity (PRA), inulin clearance, total renal blood flow (RBF), renal cortical radiomicrosphere distribution and systemic haemodynamics were examined during the first 3 h after intravenous administration of uranyl nitrate (19.9 μmol/kg body wt.; 10 mg/kg) in the dog. 2. During the 3 h of study after uranyl nitrate, urine flow remained stable or increased, sodium excretion increased approximately fivefold, renal vascular resistance increased threefold, and concordant decreases in RBF and inulin clearance to 40–50% of control values occurred. At 3 h total cortical RBF decreased to 35% of control values and the ratio of blood flow in outer to inner cortical zones also decreased, reflecting outer cortical ischaemia. PRA increased in the first hour after uranyl nitrate and slowly declined thereafter, though not to control values. 3. Respiratory rate, heart rate, mean systemic blood pressure and cardiac output were unchanged after uranyl nitrate, demonstrating that the changes in renal vascular resistance occurred without a change in peripheral vascular resistance. 4. It is postulated that increased renin-angiotensin system activity mediates the change in renal haemodynamics and the consequent fall in glomerular filtration.