Abstract Palmitate-1-14 C was infused intravenously at a constant rate for 2 to 7 1 2 hours into unanesthetized dogs, into non-obese and obese subjects with indwelling arterial and venous catheters, and O 2 uptake and CO 2 output were measured. The rate of turnover of plasma FFA was calculated from the infusion rate of the label and from the specific activity of plasma FFA. The lean subjects and the dogs were fasting 12 to 72 hours, and the plasma FFA level ranged from 0.41 to 1.6 μEq./ml. There was a straight line correlation between plasma FFA level and the turnover rate of FFA in men as well as in dogs. Calculated per Kg. body weight, dogs at the same FFA level had a turnover rate considerably higher than lean subjects, and this latter group had a higher rate than obese individuals. When the turnover rates were expressed per square meter of surface area or per caloric BMR, there was no significant difference between the lean subjects and the dogs, but obese persons showed a significantly higher turnover rate. It is concluded that mobilization of FFA in obesity is not impaired; on the contrary, it exceeds the energy requirements more than in lean persons.