A model is proposed to investigate the hypothesis that the observed time course of whole leaf photosynthetic responses to changes in incident light energy are caused by diffusional limitations. The model leaf consists of a continuously distributed mesophyll with diffusion of CO2 in the leaf interior governed by a reaction-diffusion equation. Biochemical activation is assumed to occur on a fast time scale. Both the cases of a homogeneous and a nonhomogeneous leaf interior are investigated. Using parameter estimates available in the literature, the model predicts CO2 uptake equilibration times which are much smaller than those observed. The results strongly suggest that diffusional limitations do not significantly affect photosynthetic dynamics in variable light environments.