Sequential morphological changes in tumor capillaries of istransplanted R1H rat rhabdomyosarcoma were observed weekly by transmission electron microscopy during fractionated radiotherapy (75 Gy/25 fractions/5 weeks). During the first 2 weeks of irradiation (up to 30 Gy), edema of the tumor capillary wall was induced. Swollen endothelial cells bulged into the vascular lumen and were surrounded by a widened subendothelial space with increase amounts of collagen fibrils (subendothelial edema). The endothelial lining was preserved up to the 3rd week of irradiation (45 Gy). Prolonged irradiation was associated with progressive destruction of the vascular wall including shrinkage, gradual loss of cell contacts, disappearance of the normal chromatin pattern, and increase of cytoplasmic vacuoles in endothelial cells as well as disruption of basal laminae. One week after the end of radiotherapy (75 Gy), the tumor capillaries showed complete necrosis. Progressive damage to tumor capillaries in the course of fractionated radiotherapy might have adverse effects on blood supply and thus on tumor oxygenation.