Changes in functional intravascular volume, extravasation of plasma protein, and blood flow were measured with radioisotopes in Walker carcinoma 256 of rats after 2,000 rads of x irradiation in a single exposure. The functional intravascular volume significantly decreased during the 1–11 day postirradiation period. The extravasation rate of circulating 125I-labeled plasma protein markedly increased soon after irradiation, then abruptly declined. Clearance of interstitially deposited 133Xe increased significantly 6 days after irradiation, possibly because of rapid blood flow through newly-formed arteriovenous shunts due to occlusion and stasis in the networks of long capillaries.