1. Lee Index (weight (g0.33) divided by nasoanal length (mm)), density, body water and proportional weight of the gonadal fat pad were tested as estimates of body fat in a normal population of mice and a population which included obese mice. 2. In a normal population only body water and fat-pad weight correlated significantly with body fat. 3. In a population of normal and obese mice all four indices correlated significantly with body fat. 4. Lee Index had little advantage over weight:length as an estimate of body fat, and density did not correlate well enough to be used as a method of estimation. 5. Proportional weight of the gonadal fat pad is recommended as a simple reliable estimate of body fat in normal or obese mice.