As the methanol extract of Myrica bark inhibited the activity of lipase in isolated mouse plasma in vitro, the influence of the extract on this enzyme in the gastrointestinal tube was investigated after oral intake. The methanol extract depressed the elevation of blood triglyceride level in olive oil-fed mice. Myricetin and gallic acid were suspected to be responsible for the inhibition of the lipase activity in isolated mouse plasma in vitro. The contents of myricetin and gallic acid in the methanol extract seemed to be insufficient for the appearance of the depressive activity of the extract on lipase in the gastrointestinal tract in vivo. Myricitrin, which is present in abundance in the methanol extract (about 61%), did not show the inhibitory activity. However, this material suppressed the elevation of blood triglyceride level to a slightly smaller extent compared with the methanol extract by oral administration. Consequently, it was proposed that myricitrin may function as a prodrug, which reveals its activity after conversion to myricetin, the active substance, by enterobacilli in the gastrointestinal tract.