The microvasculature of C3H/Bi mouse mammary carcinomas 8 mm in average diameter was evaluated using morphometric methods following six consecutive daily exposures of 500 R of x radiation. Tumor volume did not change significantly during the intervals between treatments. Tumors began to grow again 72 hours after the sixth 550-R fraction, with mean vessel length and surface area reaching maximum values and vessel diameters and volumes becoming minimal at this time. These changes were attributed to improved colloidal-carbon filling of previously existing nonfunctional vessles. When anatomically derived estimates of vascular dimensions were related to a metabolically useful blood supply, improved capability for exchange of essential nutrients occurred 72 hours after the sixth 500-R fraction.