A quantitative microangiographic study of renal allografts in rabbits was carried out to determine the pathogenesis of hyperacute rejection. A group of 10 rabbits, presensitized with multiple ear skin grafts from the kidney donor before receiving renal allografts, developed hyperacute rejection. Another 10 rabbits that were not presensitized served as a control group. The kidney grafts, excised at 2, 4, 8, 16 and 24 hr after renal transplantation, were examined histopathologically and microangiographically. In the presensitized group, striking vasoconstriction of afferent arterioles and interlobular arteries occurred within 2 hr after renal transplantation (p less than 0.01), but these vessels gradually recovered to a near-normal state after 24 hr. The number of glomeruli, however, began to decrease at 8 to 16 hours after renal transplantation (p less than 0.01), suggesting that the vasoconstriction produced thrombosis in the glomerular capillaries. These findings suggest that vasoconstriction at the level of afferent arterioles and interlobular arteries is the primary cause of hyperacute rejection.