1. Rat sucklings (18-d-old) bred from vitamin B12-deprived dams were compared with vitamin B12-suplemented dams’offspring, which were considered normal rat sucklings.2. The vitamin B12-deficient rat sucklings had lower body-weight, liver weight and brain weight.3. Vitamin B12deficiency was also evident from the tenfold lower concentrations of vitamin B12in liver and cerebellum.4. The concentration of liver lipid was markedly increased in vitamin B12-deficient rats; triacylglycerol accounted for most of the increase. In brain the lipid concentration was slightly decreased (P< 0.05).5. The methylation of ethanolamine phosphoglyceride to choline phosphoglyceride was reduced in both liver and brain in vitamin B12-deficient rats, as measured after the administration of [14C]ethanolamine. A slight decrease in choline phosphoglyceride concentration could be a consequence of this finding. The composition of phospholipids was otherwise normal.6. Odd-chain fatty acids (pentadecanoate and heptadecanoate) accumulated in both liver and brain of the vitamin B12-deficient rat sucklings and constituted approximately 1% of total fatty acid.7. The biosynthesis of fatty acid and cholesterol from intraperitoneally-injected3H2O and [14C]propionate was unchanged in vitamin B12deficiency.