40 h after an acute dose of CCl4 (11.3 mmol/kg), the incorporation of [1-3H]ethanolamine into rat hepatic microsomal phospholipids was inhibited to 70% of control. Incorporation into phospholipids of the inner and outer mitochondrial membranes was 30-35% of control. Rates of incorporation were equal to or above normal rates in all membranes 65 h after dosage. The activity of methyltransferase in microsomal fractions isolated from rats 10 to 66 h after dosage was depressed. These data suggest that the alteration of mitochondrial phospholipids that parallels mitochondrial dysfunction after acute CCl4 dosage could be attributed to a CCl4-induced inhibition of the microsomal phospholipid biosynthetic pathways.