Male Wistar rats of bodyweight 120 to 150 g were starved for 24 h and then given intraperitoneally L-tryptophan 10 or 50 mg/100 g bodyweight or saline. Rates of fatty acid synthesis were estimated with tritiated water or acetate-1-14C given intraperitoneally. Incorporation of 3H and 14C into fatty acids in liver was twice as much with tryptophan as with saline. Tryptophan, compared with saline, increased 1.2- to 1.8-fold the concentration in blood of pyruvate and lactate but decreased that of acetoacetate and 3-hydroxybutrate; in liver it increased significantly concentration of citrate, malate, isocitrate, glutamate and aspartate and decreased that of phosphoenolpyruvate. The rate of fatty acid synthesis in the liver was closely related to the content of citrate in the liver. Hepatic pyruvate dehydrogenase was converted into an inactive form by tryptophan; the extent of activity of the enzyme was related to the increase in fatty acid synthesis and accumulation of intermediates of the tricarboxylic acid cycle.