To understand memory-enhancing effect of red ginseng biotransformed by Bifidobacterium longum H-1 (RGB), which more potently improved scopolamine-induced learning deficit than red ginseng in the preliminary experiment, its main constituents, ginsenosides Rb1, Rg3 and Rh2, were isolated and their memory-enhancing effects investigated in scopolamine-treated mice by using passive avoidance and Y-maze tests. Among them, ginsenoside Rh2 most potently reversed memory impairment caused by scopolamine. Ginsenoside Rh2 also significantly shortened the escape latencies prolonged by scopolamine in the Morris water maze test (p<0.001) and increased the swimming time shorten by scopolamine within the platform quadrant (p<0.05). The ginsenoside Rh2 (3 muM) reversed scopolamine (10 muM)-induced suppression of long-term potentiation. It recovered field excitatory post synaptic potential (fEPSP) amplitude potentiation to 152.3+/-8.7% of the control (p<0.05). Based on these findings, RGB and its main constituent, ginsenoside Rh2, might improve learning deficits. Also the memory-enhancing effects of RGB may be dependent on the content of ginsenoside Rh2.