Mice infected with 30 cercariae of Schistosoma mansoni showed extensive destruction, obstruction, and distortion of the intrahepatic portal vein system 10 weeks after exposure. One month after curative treatment for schistosomiasis, these lesions had considerably regressed. The vascular pattern was almost normal by 2 months and completely normal by 6 months following treatment. The techniques used to study vascular changes included plastic casting, injection of colored gelatin, and conventional histopathologic methods. Main transformations observed after treatment were shrinkage of periovular granulomas and formation of new vascular channels around them, which restored the continuity of the portal vein system. Shrunken granulomas remained avascular.