The C4 acid cycle functions in Ca photosynthesis to concentrate CO2 in bundle sheath (BS) cells, thereby minimising oxygenase activity of Rubisco and associated photorespiration. Some key features of this process are reviewed here. During steady-state photosynthesis, the CO2 concentration maintained in BS cells must be a compromise since, at the higher levels required to prevent photorespiration, the potential for CO2 leakage from BS cells would be increased. Despite its central importance in C4 photosynthesis, it has not been possible to precisely determine the BS CO2 concentration. Leakage requires that the C4 cycle rate exceeds the net photosynthesis rate and lowers the efficiency of the overall process. Leakage of CO2 from the BS has been estimated by a number of indirect and, recently, by more direct methods. In a simulation, simple relationships between Rubisco activity, photorespiration, and leakage were calculated at increasing BS CO2 concentrations. From this, and determined values for leakage, the likely concentration of CO2 in BS cells may be 10-20-fold greater than in mesophyll cells. Higher concentrations would have little further effect on oxygenase activity.