A simple apparatus has been developed whereby the extent of stomatal opening on isolated epidermal strips can be monitored by measuring the flow rate of bathing solution through the stomata. This apparatus has been used to study the opening and closing response of stomata to various treatments. A supply of ions, particularly K+, was found necessary to initiate and maintain the opening of stomata in the light. In the presence of K+ stomata could be opened in the light and closed in the dark. This cycle could be repeated. The pattern of stomatal opening in the light and closing in the dark showed a similar shape with time to that shown for stomata on leaves. Similar results could be obtained in a bathing medium consisting of KH2P04-Na2HP04 buffer. The best pH found for opening was 8� O. No specificity of stomatal opening was found for the anion associated with K +. The addition of Ca2+ or Mg2+ to the bathing medium caused reductions in aperture. The extent of stomatal opening in response to the concentration of K+ in the range 0-10 mM gave a saturation curve with a concentration for half maximal opening of O� 32 mM. Increasing the concentration of K+ past 10 mM resulted in reductions of aperture.