Plasma free fatty acid (FFA) turnover rates have been estimated in dogs by a technique involving measurement of FFA specific activities during constant intravenous infusion of trace amounts of C¹⁴-labeled palmitic acid. In order to determine the relationship between FFA concentration and turnover, variations in plasma FFA levels ranging from 0.081 to 3.31 µEq/ml were induced by a variety of physiological and pharmacological treatments. Calculated FFA turnover rates ranged from 2.1 to 58.8 µEq/kg/min, with a highly significant linear regression of FFA turnover on FFA level. It is concluded that under a variety of conditions changes in FFA concentration are brought about by changes in FFA production rate and that changes in FFA uptake are simple mass-action effects of changes in FFA concentration. Respiratory C¹⁴O₂ data are presented indicating that about one-fourth of the total expired CO₂ is derived from FFA in the postabsorptive state. This accounts for the immediate fate of about one-fourth of the total FFA leaving the plasma.