Clearing-factor lipase was assayed in acetone-ether-dried powders of heart and epididymal fat-pads of lean and genetically obese mice (ob/ob). In both tissues the enzyme activity in the adult was higher in the obese mice. 2. In heart the enzyme activity was unchanged from 8 to 48 weeks of age in lean mice, but in obese mice it increased between 8 and 12 weeks of age and remained elevated. 3. Starvation produced changes in the heart clearing-factor lipase activity in obese, but not lean, mice. 4. The clearing-factor lipase activity of epididymal fat-pads decreased rapidly during 24h starvation in both lean and obese mice, but the activity in the obese mice remained higher than that in lean mice. 5. Plasma triglyceride and cholesterol concentrations were determined in both lean and obese mice. Triglyceride concentrations were not greatly different, but the obese mice were hypercholesterolaemic. Plasma cholesterol concentrations were not correlated with changes in clearing-factor lipase activity.
