The effect of octanoate and palmitate on the oxidation of 14C-labelled valine has been studied in perfused hindquarter of rat. 1. The oxidation rate of valine increases 30 times when the concentration of valine is increased from 0.1 mM to 5 mM. 2. Octanoate at a 5 mM concentration effected a 10-fold increase in the flux through the alpha-ketoisovalerate dehydrogenase step and a 5-fold increase at 0.5 mM concentration. 3. Palmitate (1 mM) effects only a moderate increase in the valine oxidation. 4. With no octanoate there was a great accumulation of alpha-ketoisovalerate in both the muscle and the perfusion medium. 5. With octanoate little alpha-ketoisovalerate accumulated whereas 3-hydroxyisobutyrate was found in high concentration both in the muscle and in the medium. 6. Octanoate stimulated the production of citric-acid-cycle intermediates and lactate. 7. The results are discussed in relation to valine metabolism in the body.