We previously found that hepatic lipogenic enzyme in-duction, fatty acid synthesis, and triglyceride level were markedly lower in rats fed soybean protein than in those fed casein (Iritani et al. (1986): J. Nutr., 116: 190). After labeling triglycerides with tritiated water, the effects of dietary protein on the triglyceride degradation have been investigated. After the injection of tritiated water into rats, the radioactivities of fatty acids and triglycerides reached a plateau in 1-2 days and were markedly lower in the soybean protein group than in the casein group. The decreasing rates of triglyceride radioactivities were similar between the casein and soybean protein groups. The enzyme activities in glycerolipid synthesis were similar between the groups. Therefore, the lowering effects of soybean protein on triglyceride levels appear to be ascribed to triglyceride synthesis (due to fatty acid synthesis) rather than to the degradation.