Enzymatic synthesis of glycerolipides involves the formation of ester bonds between glycerol and fatty acids or phosphoric acid. The similarity among the structures of the different glycerolipides speaks in favor of common metabolic pathways and interchange of major parts of their molecules. Recently Kennedy and coworkers (1, 2) have shown that preparations of rat liver mitochondria can synthesize both lecithins and triglycerides from added diglyceride mixtures. Consequently, intact tissues may possibly utilize diglycerides as the common intermediates in the synthesis of these glycerolipides. If this were the case, a predictable relationship should be produced between the patterns of radioactive isotope incorporation in triglycerides and phosphoglycerides. Accordingly, radiocarbon-labeled precursors were incubated with sliced tissues and the lipides were isolated in order to compare the amounts of isotope in each type of lipide. The results of these experiments suggest that the diglyceride unit in the phospholipides is metabolically different from that in the triglycerides.