It is supposed that oscillations in stomatal conductance are associated with the dynamic properties of the loop in which rate of evaporation affects, through physiological processes, the aperture of stomata and stomatal aperture in turn affects rate of evaporation. It is therefore predicted that their occurrence must be influenced by the magnitude of what is termed environmental gain: the sensitivity of rate of evaporation to change in leaf conductance to vapor transfer. Two methods of manipulating gain, and their effects on stomatal behavior in cotton (Gossypium hirsutum L. cv. Deltapine Smooth Leaf), are described. In the first, gain was increased by decreasing ambient humidity; in the second, it was made zero by regulating ambient humidity to keep rate of evaporation constant despite changes in conductance. The results are in accord with the supposition.