Oxygen consumption was measured in a group of 18 grossly obese females; creatinine excretion in 16; and total body water and exchangeable potassium in 14 members of this group. Body fat, surface area and body weight showed high coefficients of correlation with oxygen consumption whereas, measures of lean body mass such as total body water, exchangeable potassium, and creatinine had much less satisfactory correlation coefficients. With increasing age, oxygen consumption per unit surface area showed a small decline