In liver, two pathways for biosynthesis of phosphatidylcholine are presented ; One is the stepwise methylation pathway from phosphatidylethanolamine and the other via phosphorylcholine and cytidine diphosphate-choline (CDP-choline). To investigate the dynamics of these pathways, the incorporation of ^3H-methyl-choline chloride and 1,2-^<14>C-ethanolamine into male rat liver microsomal phosphatidylcholine were examined under different metabolic conditions ; diet devoid of cobalamin fed, CCl_4 injured and methylcobalamin treated livers and nervous systems. Phosphatidylcholine biosynthesized by the pathway via CDP-choline mainly contributes the intertransportation of phosphatidylcholine between microsome and mitochondria, and constructs the mitochondrial phospholipidsprotein complex. The rate of transmethylation of phosphatidylethanolamine was easily affected by several biological conditions. The phosphatidylcholine synthesized from phosphatidylethanolamine was increased in methylcobalamin treated group, but not so much provided with the transportations between mitochondria and microsome and have the different biological function from that of phosphatidylcholine via CDP-choline.