Ginsenosides (saponins extracted from Panax ginseng) elicit qualitatively and quantitatively different responses in isolated, contracted ring preparations of different blood vessels from rabbits, dogs and humans. Ginsenosides themselves did not affect the tone of 'resting' isolated blood vessels directly, but contracted slightly the renal vein of rabbits at the maximum concentration tested. The mixture caused relaxation of the noradrenaline (NA) or prostaglandin F2 alpha (PGF2 alpha)-induced contraction of the pulmonary artery and intrapulmonary artery of rabbits, and the PGF 2 alpha-induced contraction of the canine mesenteric vein. Ginsenosides potentiated, in a concentration-dependent manner, the contractile responses of renal veins of dogs and rabbits to PGF2 alpha The reason for such heterogeneous responses of different blood vessels to ginsenosides in unknown. It is suggested that either potentiation of contraction or relaxation of contracted blood vessels might be mediated by interaction with endogenous vasoactive substances. The potentiation of PGF2 alpha-induced contraction may be related to the reduction of renal blood flow observed in anaesthetized dogs. The simultaneous contraction and relaxation effects may explain its biphasic actions on blood pressure.