Structural changes in irradiated tumors and their regrowth were studied in a rat hepatoma, AH109A, using histological and transparent-chamber techniques. The development of the tumour was examined by means of vascular morphometry as observed in the chamber. Schematically, the tumour tissue was divided into four isocentric layers according to vascular morphology and measurements of vessel volume, surface area, and length per mm3 of tissue. The vascularity was greatest in the outermost region, decreased towards the inner parts and reached an absence of vascularity at the central necrosis. The tumors were gamma- or X-irradiated with various doses. The inside hypoxic region was destroyed completely after 3000 rad, and regrowths started exclusively from the outermost area of the tumour where enhancement of the effect of radiation by oxygen was thought to be greatest. Possible mechanisms of tumour regrowth are discussed.