1. The [32P]phosphate incorporated into the phospholipids of isolated rat hepatic cells is present in phosphatidic acid and to a smaller extent in phosphatidylinositol. 2. The ability to synthesize nitrogen-containing phospholipids is restored by adding a liver supernatant fraction, and it is suggested that the metabolic deficiency is caused by the leakage of cytoplasmic enzymes of the synthetase system from the cells. 3. Fortified cell preparations were pulse-labelled with [32P]phosphate, [Me-14C]choline, [2-14C]ethanolamine and [U-14C]inositol and the subsequent fate of the labelled microsomal and mitochondrial phospholipids followed. 4. A fall in the specific radioactivity of microsomal phospholipids and a rise in that of mitochondrial phospholipids is interpreted as providing evidence of a transfer of labelled phospholipid molecules from the synthetic site (endoplasmic reticulum) to the mitochondrial membranes in the intact cells. 5. The formation of the phospholipids of mitochondrial membranes is discussed.

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