Monocrotaline, an alkaloid obtained from Crotalaria spectabilis, provides an experimental model for studies of mesangiolysis as seen in Habu venom poisoning and hemolytic-uremic syndrome. Renal lesions induced in rats by a single intraperitoneal administration of monocrotaline were examined by light and electron microscopy. Two weeks after the injection glomerular capillaries showed focal endothelial cell detachment and adhesion of platelets to the basal lamina. At 3 weeks there was no endothelial denudation but the mesangium was severely edematous. Mesangiolysis occurred at 4 weeks, accompanied by dilatation or obliteration of capillaries and necrosis and hemorrhage in the mesangium. Insudation of plasma constituents facilitated by the endothelial injury appeared important in the pathogenesis of mesangiolysis. Fusion of capillaries seemed to lead to capillary aneurysmata and necrosis of the mesangium. These glomerular lesions induced by monocrotaline were similar to those of Habu venom poisoning or hemolytic-uremic syndrome.