The activity of lactate dehydrogenase and fractions of this enzyme has been estimated in human large saphenous veins. Typical varicosities were compared in composition with normal veins from the same patients. The total LDH activity was significantly decreased in typical varicosities. Varicose veins further differed from macroscopically normal veins by a lower content of aerobic fractions of LDH and a higher content of anaerobic fractions. These results suggest that there is a disturbance in metabolism of the smooth muscle of the venous wall in varicosities which would make the diseased wall more dependent on anaerobic glycolysis than normal tissue.