Whatever the signals which trigger the repair process, it is clear that the rate of healing is dependent on the local delivery of O2 and other nutrients. New tissue growth is centered around a blood vessel sprouting new capillaries to supply the advancing cells. The endothelial buds grow towards areas of low O2 tension but they will not do so unless preceded by macrophage invasion. Macrophages probably act as director cells to provide a chemotactic signal for endothelium to follow and release stimulatory substances for fibroblast formation and activation. The synthesis of collagen by fibroblasts depends on the availability of molecular O2. The main function of polymorphonuclear leukocytes in the wound is to resist infection. An important mechanism by which white cells selectively kill bacteria uses O2. Any treatment that augments the local O2 supply or helps to avoid hypoperfusion of the wound will increase the rate of healing and decrease the susceptibility to infection. Rats were used to study wound healing.