Labeled 14-methylhexadecanoic acid was administered to normal rats, animals bearing the Walker 256 tumor at various stages of its growth and to tumor-resistant rats and its distribution in lipids (free fatty acids, triglycerides, phospholipids and cholesteryl esters) was studied during 15 min to 3 hours following its injection in the liver, blood and tumor tissue. The period of the most active tumor growth was associated with a significantly better utilization of this fatty acid for the synthesis of lipids. The quantities of radioactive triglycerides, phospholipids and cholesteryl ester in the liver were highly increased during this time. In tumor-resistant animals the levels of radioactive lipids were similar to those in control normal animals but the turn-over of 14-methylhexadecanoic acid was considerably slower than in controls. The turn-over of the cholesteryl 14-methylhexadecanoate in the liver was significantly changed at the late stages of the tumor growth. The level of this cholesteryl ester in the blood decreased progressively during the tumor growth whereas that of triglycerides increased. No significant changes were associated with the free fatty acid and phospholipids in the blood. The total radioactivity present in the tumor increased from 0.05% up to nearly 1% of the administered 14-methylhexadecanoic acid during the growth of the Walker 256 tumor. The level of radioactive cholesteryl 14-methylhexadecanoate in the tumor tissue increased progressively during 3 hours following the injection of 14-methylhexadecanoic acid. Thus the tumor growth is apparently accompanied by significant changes in the metabolism of 14-methylhexadecanoic acid and results in an enhanced synthesis of lipids in the liver tissue. The newly synthesized lipids, in particular cholesteryl 14-methylhexadecanoate, are transported in the blood stream into the tumor where they accumulate.