Measurements were made of pressures applied in subsidiary and guard cells which caused closure and opening of stomatal pores. These pressures were lower than would be expected from plasmolytically determined osmotic potentials of guard cell saps. The pressures needed in guard cells to open almost closed stomata were practically the same as those required to close open stomata when applied in adjacent subsidiary cells. Completely closed stomata could not be opened by this technique. The implications for the understanding of the mechanism of guard cell deformation, the Spannungsphase, and the pressure potentials of guard cells are discussed.