Effects of four citrus flavonoids, hesperidin, naringin and their aglycones on phosphatidate phosphohydrolase(PAP,EC 3.1.3.3) activity were examined using isolated rat microsomes as an enzyme source. In addition, these flavonoids were tested to see whether they exert any influence on the proliferation and growth in cultured human hepatocytes HepG2 cells. Flavonoids at concentration up to 10^(-4)M had no significant effect on the number of cells and cell proliferation by MTT cell growth assay method, whereas aglycone flavonoids, hesperetin and narigenin, at concentration of 10^(-3)M significantly inhibited cell proliferation. Hesperetin inhibited PAP activity in a dose-dependent manner starting at concentration of 10^(-5)M. Narigenin at concentration of 10^(-2)M inhibited PAP activity markedly, while the other flavonoids did not show any significant effect. The present study, therefore, demonstrated that aglycone flavonoids exerted portent effects on PAP activity and on cell proliferation.