The relationship between CO2 concentration and starch synthesis and degradation was studied by measuring leaf starch content and disappearance of14C-starch. At a concentration of 340 microliters CO2 per liter, starch accumulated without degradation of previously synthesized starch. Degradation of starch began when CO2 concentration was lowered, but its synthesis continued. At 120 microliters CO2 per liter rates of synthesis and degradation were equal. Even at the CO2 compensation point, synthesis of starch continued. Concomitant starch synthesis and mobilization supported export from the leaf. Changes in starch metabolism that occur when photosynthesis is CO2-limited provide a means to study regulation of starch metabolism and carbon allocation in translocating leaves.