The present study was performed to clarify whether the crude saponins from stems and leaves of Panax quinquefolium inhibited lipase activity in vitro, and prevented obesity induced in mice by feeding a high-fat diet for 8 weeks. For in vitro experiments, assay for the inhibitory effects of saponins from stems and leaves of Panax quinquefolium on pancreatic lipase activity was performed by measuring the rate of release of oleic acid from triolein. For in vivo experiments, female ICR mice were fed a high-fat diet with or without saponins from stems and leaves of Panax quinquefolium for 8 weeks. The crude saponins inhibited pancreatic lipase activity in vitro. Furthermore, crude saponins (lg/kg body weight) inhibited the elevations of plasma triacylglycerol in rats administered the oral lipid emulsion tolerance test. In addition, long-term administration of crude saponins, the parametrial adipose tissue weight was decreased by feeding a high-fat diet containing l% or 3% crude saponins compared to those of high-fat diet group. It is demonstrated that the anti-obesity effects of the crude saponins from stems and leaves of Panax quinquefolium in high-fat diet-treated mice may be due to the inhibition of intestinal absorption of dietary fat by ginsenosides Rc, Rb(1) and Rb(2).