The fatty acid composition of Ehrilich ascites tumor cell nuclei was differend when the tumor-bearing mice were fed diets rich in either coconut or sunflower oil. When coconut oil was fed, the monoenoic fatty acid content of many of the nuclear lipids was increased and their polyenoic fatty acid content was reduced as compared with the sunflower oil diet. By contrast, only small changes were produced in the saturated fatty acid contents of the nuclear lipids. The nuclear membrane choline phospholipid, ethanolamine phospholipid and combined serine phospholipid plus inositol phospholipid fractions exhibited statistically significant changes in fatty acid composition, but the sphingomyelins were not altered appreciably by dietary lipid modification. The fatty acid composition of the small quantity of phospholipids associated with the chromatin was much more resistant to diet-induced mosification. Except for sphingomyelin, the fatty acid composition of the chromatin phospholipids was different from that of the corresponding nuclear membrane phospholipids, containing much larger amounts of fatty acids having less than 16 carbon atoms. The fatty acid compositons of the nuclear triaclglycerols and cholesterol esters, which were associated almost entirely with the chromatin, were modified by the dietary lipid modifications. There were no changes in the DNA, RNA or lipid content of these nuclei. Therefore, this experimental system can be used to prepare mamalian nuclei that differ appreciably only in their fatty acyl composition.