Oxygen uptake of aortas of young rats is higher than that of old rats in KRP solution. In serum of young rats, oxygen uptake and especially glucose uptake is higher than in serum of old rats in either young or old aortas. Anaerobic formation of lactic acid by the rat aorta is greater in serum of old rats than in serum of young rats. A fat-rich diet with 55 per cent butter fat, given for 6 weeks, significantly decreases oxygen and glucose uptake. This effect is not seen if soya oil is used instead of butter fat. A cholesterol-rich, atherosclerosis-producing diet decreases the glucose uptake. Norepinephrine, in vitro and in vivo, increases the oxygen and glucose uptake and the glycogen synthesis; epinephrine has no influence; acetylcholine decreases the oxygen and glucose uptake. The oxygen and glucose uptakes of aortic tissue from alloxandiabetic rats are lower than those from normal rats. Insulin in vitro increases oxygen and glucose uptake of the aorta in a Gey and Gey medium, this effect being greater in aorta of old rats.