(1) [1-(14)C]Palmitic acid was oxidized to CO(2) and a water-soluble material by a rat brain preparation. The radioactive CO(2) and water-soluble material were produced in a ratio of 1.0:1.3 when the mitochondrial fraction was used, and 1.0:10 or more with the postnuclear fraction. There was a lag period of 10 min for CO(2) production. These conversions were stimulated by carnitine and inhibited by cyanide. (2) Of the total radioactivity in the water-soluble material obtained with the mitochondrial fraction, 65% after 10 min of incubation and 80% thereafter were associated with amino acids, mostly with aspartate and glutamate. The remaining radioactivity, 35 and 20%, respectively, was associated with organic acids, 60-65% in citrate. The water-soluble material obtained with the postnuclear fraction contained an equal amount of radioactivity in organic and amino acids during the course of the experiment. In the organic acids, succinate was the highest labeled product during 10-40 min of incubation, whereas citrate was the highest labeled at the end of 60 min of incubation. After 60 min, the radioactivity in the amino acids was markedly associated with glutamate, and its radioactivity was 10 times greater with the postnuclear fraction than with the mitochondrial one. (3) An experiment with rat live preparations was also carried out. The liver mitochondrial fraction showed an accumulation of radioactive organic acids within 10 min of incubation, which was followed by a linear production of (14)CO(2). With the liver postnuclear fraction, the radioactivity was found mostly in the organic acids during the course of the experiment. In the liver system, the radioactive amino acids accounted for only 25% or less of the total radioactivity in the water-soluble material.