Primary rat hepatocyte cultures exposed to tetradecylthioacetic acid for periods up to 96 h significantly increased fatty acid oxidation and decreased triacylglycerol synthesis and secretion. During the same period the mean areal fraction (%) and polydispersity of both mitochondria and peroxisomes increased, indicating growth and proliferation. In rats fed tetradecylthioacetic acid for 12 weeks, the fatty acid oxidation increased with a concomitant hypolipidemic effect. In addition, the areal fraction of both mitochondria and peroxisomes increased significantly and the number of lipid droplets decreased. The results suggest that tetradecylthioacetic acid affects mitochondria and peroxisomes both in vitro and in vivo. It is concluded that tetradecylthioacetic acid reduces secretion of triacylglycerol from rat hepatocytes both in vitro and in vivo mainly by stimulating fatty acid oxidation.