Cyclic adenosine monophosphate (AMP) phosphodiesterase inhibitors contained in Polygala tenuifolia WILLD. were identified as saponins and oleic acid. The concentrations of onjisaponins E, F and G required to give 50% inhibition (IC 50) were of the sameor der as that of papaverine. A kinetic study revealed that onjisaponin F acts non-competitively against cyclic AMP phosphodiesterase, like papaverine. Onjisaponin F exhibited a prolongation effect on hexobarbital sleeping time in mice.