Ginsenosides, the main component of Panax ginseng root, have been reported to show several pharmacological actions on the peripheral metabolism of glucose and lipid and on endocrine secretion. The present study aimed to clarify the effects of ginsenoside-Rb1 on feeding behavior and endogenous chemical substances. Rb1 infusion into the rat third cerebroventricle was started at 1930 hr, and ingestive behavior was recorded in a soundproof room illuminated daily from 0800 to 2000 hr. Rb1 at doses of 0.05, 0.10 and 0.20 mumol potently decreased food intake dose-dependently during the first dark period after infusion. Analysis of meal patterns revealed that the suppressive effect was due to decreasing meal size, but not to postprandial intermeal interval and eating speed. Drinking episodes decreased concomitantly with feeding suppression only at the highest dose of 0.20 mumol. Ambulatory activity was not affected in the doses tested. Infusion of Rb1 increased plasma glucose, leaving insulin unaffected. Microinjection of 0.01 mumol Rb1 into the hypothalamic ventromedial nucleus (VMH) decreased food intake, but injection into the lateral hypothalamic area did not. Taking these data together, Rb1 was found to have a suppressive effect on feeding partly through the VMH.