Effects of hesperetin and naringenin on the concentration of triacylglycerol in the serum and liver were studied in male golden hamster fed with the semipurified diet containing at 1% level of them for 3 weeks. The concentration of triacylglycerol in serum of the naringenin group decreased by 31%, whereas that in liver increased by 37% compared to the control group. The concentration of triacylglycerol in the serum and liver of the hesperetin group was slightly lower than the control group. The activity of microsomal phosphatidate phosphohydrolase in the liver, which is a key enzyme for biosynthesis of triacylglycerol, was significantly inhibited in the hesperetin group, whereas it was not affected in the naringenin group. The effect of hesperetin on phosphatidate phosphohydrolase was also measured in vitro. Hesperetin decreased the activity of phosphatidate phosphohydrolase with a dose-dependent manner. Both naringenin and hesperetin did not statistically affect the daily food consumption, body weight, liver weight, and total cholesterol in the serum. The observation accounts for the hypotriglyceridemic effect of hesperetin in the hyperlipidemic hamster.