Obesity tissue, as measured in this experiment, is composed of fat, cellular matter and extra cellular water. This conclusion was drawn from an analysis of the changes in body density and extracellular water in a group of men induced to overeat. An increase in extra cellular water, intracellular water, and total body water as well as body solids and exchangeable K was found in the group of obese females. It was inferred that cell mass and lean body mass were also larger. No significant differences were found in the intracellular K concentration, estimated for the obese and normal groups. There was no difference between the 2 groups in the interrelation of the body constituents and body weight. The excess tissue of the obese group was 22% total water, 15% intracellular water and 7% extracellular water.