The pharmacokinetic character of ginsenoside Rb₁ (Rb₁), one of the main 20 (S)-protopanaxadiol group saponins of ginseng (Panax ginseng C.A. MEYER), was investigated in rats. First, quantitative analysis of Rb₁ in biological samples was investigated and the most suitable assay procedure for each biological sample was established. Little Rb₁ was absorbed from the digestive tract after oral administration (100mg/kg) to rats. The serum level of Rb₁ in rats after intravenous injection (5 mg/kg) declined biexponentially, and the half-life of the β-phase was 14.5 h. The long persistence of Rb₁ in serum and tissues in rats after intravenous administration was assumed to correlate with the high activity of plasma protein binding. Rb₁ was gradually excreted into urine, but not bile. Unabsorbed Rb₁ in the digestive tract was rapidly decomposed and/or metabolized mainly in the large intestine. These results are quite different from our results on ginsenoside Rg₁ in rats.