Technical Abstract: Leaf water potentials below threshold values result in reduced stomatal conductance. Stomatal closure at low leaf water potentials may serve to protect against cavitation of xylem. Possible control of stomatal conductance by leaf water potential or hydraulic conductance was tested by drying the rooting medium in four herbaceous annual species until stomatal conductance was reduced, and then lowering the carbon dioxide concentration to determine whether stomatal conductance and transpiration rate could be increased and leaf water potential decreased, and whether hydraulic conductance was reduced at the resulting lower leaf water potential. In all species, low carbon dioxide concentrations could reverse the stomatal closure due to drying despite further reductions in leaf water potential, and the resulting lower leaf water potentials did not result in reductions in hydraulic conductance. Complete reopening of stomata could be induced by low carbon dioxide concentrations in leaves in which stomatal conductance at air levels of carbon dioxide were as low as about 50% of maximum. The relative sensitivity of stomatal conductance to internal carbon dioxide concentration in the leaves of dry plants of each species averaged 3 to 4 times higher than in leaves of wet plants.