Young female rats of the Wistar strain were fed a 20% soybean diet (approximately 8% soybean protein) with or without the supply of 0.3% methionine. Methionine supply caused an accumulation of triglycerides in the liver. In vitro incorporation of acetate-1-14C into the liver lipid fraction and its incorporation ratios of triglyceride/phospholipid were higher in the livers from rats fed the methionine-supplemented diet (Met diet) than in those of animals fed the 20% soybean diet (basal diet). In vivo incorporation of 32P into the phospholipid fraction was lower in the livers of rats fed the Met diet than in those of rats fed the basal diet. Injection of phospholipid (lecithin) through a tail vein increased the plasma level of esterified fatty acids in rats fed the Met diet only and not in those fed the basal diet. Injection of an apolipoprotein fraction produced no effect on the plasma esterified fatty acid levels. The effect of phospholipids in increasing triglyceride transport was confirmed in liver perfusion experiments. These results indicate that supplying methionine to the low soybean protein diet induced disproportionation of synthesis and transport of triglycerides by the shortage of phospholipids.