The measurement of plasma free fatty acid turnover and oxidation by means of the isotopic dilution technique using a radiolabeled free fatty acid has proven useful for the in vivo study of fat metabolism in animals. However, because of the ethical considerations regarding the administration of radioisotopes to humans, there is a lack of adequate information regarding the in vivo regulation of free fatty turnover and oxidation in humans, particularly children. Consequently, we have developed a technique using the stable, nonradioactive [1-13C]palmitic acid molecule as free fatty acid tracer to measure plasma free fatty acid metabolism in vivo. We have analysed the isotopic enrichment of plasma palmitate by selected ion monitoring on a gas chromatograph mass spectrometer and we have used an isotope ratio mass spectrometer to determine the much lower isotopic enrichment of the expired CO2. We have found this technique to be safe to administer, to a high degree of analytical reproducibility, and to yield values in dogs comparable with those obtained using [1-14C]palmitic acid as the tracer.