In the course of our screening work for new types of lipase inhibitors, hesperidin was identified as a simple and small molecular weight inhibitor in the peels of Citrus unshiu. Hesperidin inhibited lipase activity from porcine pancreas and that from Pseudomonas, and their IC50 was 32 and 132 micrograms/ml, respectively. Hesperidin, neohesperidin, narirutin, and naringin are known as the main flavonoids in Citrus unshiu. Neohesperidin also inhibited the lipase from procine pancreas, but did not have any effect on Pseudomonas. Narirutin and naringin did not show this effect on lipases from porcine pancreas and Pseudomonas. In animal experiments, the concentration of plasma triglyceride in rats fed a diet containing 10% hesperidin were significantly lower than that fed the control diet.