The role of the phosphate translocator and the importance of the extrachloroplastic concentrations of phosphate, 3-phosphoglycerate, and dihydroxyacetone phosphate in steady-state photosynthesis is examined with a kinetic model. The steady-state stromal concentrations of these compounds are calculated as a function of the rate of the various partial reactions of photosynthesis, at various external concentrations which span those likely to occur in vivo. It is shown how the net transport requirements of the various reactions necessitate different adjustments in the stromal concentrations of these compounds, away from the equilibrium values expected in the absence of metabolism. Under most circumstances, the high exchange capacity of the phosphate translocator relative to the transport requirements of CO(2) fixation limits the extent of these displacements, but conditions when the phosphate translocator is limiting photosynthesis are observed and discussed. The model provides a basis for a more quantitative understanding of the role of the phosphate translocator and the external concentrations of phosphate, 3-phosphoglycerate, and dihydroxyacetone phosphate in photosynthesis.