The primary effort of Gates and his colleagues at present is to describe steady-state levels of photosynthesis, but in addition a method of treating leaf diffusion resistance is desired that will be suitable for computer simulations integrating photosynthesis and transpiration. The original model for photosynthesis contained only alight dependency for stomatal resistance. Although facts concerning the field behavior of stomata are extensive and sometimes contradictory, we know little about the relative importance of a particular controlling factor. The following experimental study was undertaken to assess the relative importance of light and other environmental factors affecting leaf diffusion resistance in the field. In certain situations, simple description of diffusion resistance appears adequate and can be used in simulating transpiration rates and water use. In other situations, a model of stomatal behavior must undoubtedly approximate the living system more precisely.