The effects of 6 h of ischemia and 6 h of perfusion with 10% dextrose and water solution, Sacks'' solution and Intralipid on the endothelium of common femoral arteries in dogs were examined by light microscopy and by scanning EM and compared with normal arteries. Arteries that were ischemic or perfused with 10% dextrose and water solution or Sacks'' solution showed a flattening of the normal linear convolutions and extensive crater formation, with fragmentation and even complete loss of endothelial cells in many areas. The more severe changes occurred in perfused vessels. Fibrin and platelets covered the luminal surface in many areas in which there was extensive injury to the endothelium. Light microscopy revealed thickening of the internal elastic membrane and intimal fibrosis. Results of biopsies performed 2 wk after perfusion showed slight, although incomplete, improvement in the structure of the endothelium. Arteries perfused with Intralipid had thickening of the intima with a proliferation of plump cells oriented linearly over the surface but with an absence of craters. Injury to endothelial cells may be produced by ischemia and perfusion with various solutions may produce an even more severe alteration in structure of the endothelial cell which does not return to normal within 2 wk. The origin of new endothelial cells is not demonstrated by this study.