Local blood flow was measured in renal cortex (1 mm below cortical surface) by means of the hydrogen clearance method in urethanized rats. Recording of blood pressure from femoral artery was performed. Crotalus durissus terrificus venom injection (one mg/kg i.v.) significantly decreased cortical blood flow at 10 min, without a significant arterial pressure modification. Posterior injection of mannitol 200 mg induced a significant increase in cortical blood flow, although initial values were not reached. Electron microscopy showed thromboses in the glomerular capillaries 35 minutes after venom injection. It is suggested that the precocious effect of this venom on renal cortical blood flow may be instrumental in the development of the renal acute insufficiency induced by Crotalus durissus terrificus venom.