SUMMARY:(i) An account of the changes in calibre of vessels, Irradiated with various doses of γ‐rays from60Co., has been described, from direct microscopic observations of vessels,in vivo, using the Sandison Clark ear chamber technique.(ii) Attention has been drawn to the essential dynamic character of the reactions, with particular reference to changes in normal vasomotion.(iii) The changes have been analysed, by certain pharmacological techniques designed to test the functional integrity of irradiated vascular smooth muscle.(iv) Descriptions are given of “immediate” vascular dilation the primary roentgen erythema and the secondary roentgen reaction, and the factors responsible are analysed.(v) The observations have been made on vessels “isolated” from epithelial proximity, and the indirect effects of epithelial damage on vascular reactions are discussed.(vi) The fallacy which underlies the conception of radiation, erythema as a static dilation of blood vessels, is discussed, in view of the