1. Leaves were exposed to 14CO2 under steady-state conditions for photosynthesis. The kinetics of entry or loss of label in pools of CO2 and other compounds was examined during the period of the pulse and a ‘chase’ with 12CO2. 2. With maize the kinetics of labelling of the major CO2 pool and of depletion of label during a ‘chase’ was consistent with this pool being derived from the C-4 of malate and being the precursor of the C-1 of 3-phosphoglycerate. 3. Similar results were obtained for Amaranthus leaves except that the C-4 of aspartate rather than malate was apparently the primary source of CO2. 4. The size and turnover time of the CO2 and C4 acid pools was calculated. These results provided the basis for estimating the concentration of CO2 in the bundle-sheath cells or chloroplasts assuming the pool was largely restricted to one or other of these compartments. 5. These findings are considered in relation to current schemes for the C4-pathway and the operation of a CO2 concentrating mechanism to serve ribulose diphosphate carboxylase.

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