When fasted rats were fed fat-free diets containing various sources of protein for 3 d, the activities of liver glucose-6-phosphate dehydrogenase, malic enzyme, acetyl-CoA carboxylase and fatty acid synthetase were markedly lower in rats fed soybean protein or gluten than in those fed casein or fish protein. Since malic enzyme mRNA activity was not low in the soybean protein-fed animals, the translation of malic enzyme appears to be suppressed by dietary soybean protein. The incorporation of tritiated water into liver fatty acids was significantly lower in animals fed soybean protein than in those fed casein. The triglyceride levels in plasma and especially in liver were also lower in the groups fed soybean and gluten than in the groups fed casein and fish. In addition, when dietary soybean protein was replaced with amino acids to simulate casein or soybean protein, the effects on the levels of lipogenic enzymes were still found but were not as great. Thus, some effects can be ascribed to the protein itself and some to the amino acid composition of the diet.