Heat-induced spontaneous contraction of blood vessels as observed during the application of surgical lasers was investigated experimentally. By means of extravascular irrigation of veins and arteries from the rabbit mesentery in vivo with saline that had been preheated to varying temperatures in the range of 45-90 degrees C, the temperature dependence on contraction with evaluated. It was demonstrated that lower temperature thresholds of approximately 70 degrees C and 75 degrees C exist for the shrinkage of veins and arteries, respectively. Ultrastructural investigation of the walls of contracted and of noncontracted vessels indicates that vessel contraction correlates with the denaturation of collagen fibrils. It was therefore concluded that the well-known heat shrinkage effect of denatured collagen accounts for the heat-induced vessel constriction. It is shown that on heat-induced permanent occlusion, vessels of the type that were examined cannot be achieved reliably at temperatures of up to 90 degrees C for 16 sec; in most cases relaxation takes place.