The response of mouse blood vessels to radiation was studied by locally irradiating mesenteric vessels, in an exteriorized segment, using a 90Sr .beta. applicator. Four different assays of damage were made when the animals were sacrificed from 6 wk to 18 mo. after irradiation. In all studies sham-irradiated animals were compared with those receiving 20, 30, and 45 Gy [gray]. Vein diameter was studied by direct observation, vascular volume and permeability by isotope techniques, and the cellularity of the arterioles from histological sections. Within 3 mo. changes were seen in all these parameters, with a gradual recovery toward normal by 12 mo., an a 2nd phase of damage appearing at 18 mo. The time course of changes in vascular permeability appeared to match the loss of endothelial cells lining the arterioles, but there was a significant correlation only at 6 wk. Fibrosis was not seen before 12 mo., but at 18 mo. the vessel walls were fibrosed, sclerotic, and occasionally the lumen was occluded. At no time was any change in the dependent ileum seen. The blood supply to this tissue was maintained in spite of the severe radiation damage in the vessels.