1. 1. The livers of male rats were exposed to [ Me- 3 H]choline, [1,2- 3H]ethanol-amine, or [ Me- 3 H]methionine by intraportal injection for periods varying from 15 s to 2 h. 2. 2. The incorporation of choline into hepatic phosphatidylcholines was predominantly by phosphorylation, utilized CDPcholine as an intermediate, and favored the more saturated subfractions of lecithin; whereas choline-exchange was negligible. The oxidation of choline to betaine was at least equal to the phosphorylation of choline. Betaine was rapidly demethylated and the methyl groups reutilized. 3. 3. The incorporation of ethanolamine into hepatic phosphatidylethanol-amines was predominantly by phosphorylation and utilized CDPethanolamine as an intermediate; whereas ethanolamine-exchange was negligible. 4. 4. After the injection of [1,2- 3H]ethanolamine, radioactive maxima were observed in phosphorylcholine and CDPcholine prior to that in phosphatidylcholine, indicating that phosphorylethanolamine and/or CDPethanolamine were substrates for methylation. 5. 5. After the injection of methionine, radioactive maxima were observed in choline, betaine, and phosphorylcholine well before that in phosphatidylcholine, indicating their direct synthesis by the methylation of ethanolamine and phosphoryl-ethanolamine. 6. 6. Tritium from either [1,2- 3H]ethanolamine or [ Me- 3 H]methionine was preferentially incorporated into the more unsaturated subfractions of phosphatidylcholine.