The effect of Daflon 500 mg, consisting of 90% diosmin and 10% hesperidin, on bovine veins and lymphatic vessels was shown. The recording of mechanical tension on metacarpal vein rings demonstrated a dose-effect relationship with an ED50 of 0.5 X 10(-6) M. This effect observed on isolated organs, is close to the tonic effect of Daflon 500 mg observed in human. An effect was also observed as demonstrated by the depolarisation curve obtained after administration of solutions containing increasing concentrations of potassium. The curve was shifted to the right which may indicate a decrease in potassium permeability in the presence of Daflon 500 mg. On lymphatic vessels isolated from mesenterium, the main effect was an increase in the frequency of spontaneous response which could explain the effect of Daflon 500 mg on lymph flow as shown previously. This dose-effect relationship could be due to an increase in the sensitivity of receptors to some agonists. However, this effect could be pleiotropic. This pleiotropic effect could involve the stimulation by Daflon 500 mg of other cellular mechanisms such as phosphatidylinositol metabolism, or more directly, the pacemaker activity of the cells which govern spontaneous activity, pacemaker activity being under the control of membrane potassium permeability. Some evidence about the action of Daflon 500 mg on potassium permeability could also be shown on other tissues.