We have assessed the effect of vanadate as an inhibitor of plasma-membrane ATPase on photosynthesis and the ATP/ADP ratio in Chlamydomonas reinhardtii CW-92 (a mutant strain lacking a cell wall). This effect was compared in low-CO2-adapted cells grown in media bubbled with air containing 400 or 70 gL.L CO 2. Evidence is presented indicating that cells grown at 70 laL'L J CO2 have a higher rate of photosynthetic 02 evolution than cells grown at 400 ~tL. L-1 CO2, at limiting carbon concentrations. Extracellular and intracellular carbonic-anhydrase activities were, however, similar in cells grown in both of the low-carbon conditions. Vanadate inhibited, to a different extent, the HCO3-dependent 02 evolution in cells grown at 400 and 70 gL. L ~ CO2. At 400 gM vanadate, inhibition reached 70-75% in cells grown at 400 gL.L 1 but only 50% in those grown at 70 pL.L i CO2. The ATP/ADP ratios determined with and without vanadate at limiting concentrations of dissolved inorganic carbon indicated that more ATP was hydrolysed in algae grown at 70 pL. Lthan in those grown at 400 gL.L i CO2. We conclude that the maximal capacity to accumulate dissolved inorganic carbon is inversely related to the CO2 concentration in the medium. Activation and or synthesis of vanadate-sensitive ATPase may be the major explanation for the higher capacity for HCO3-dependent O2 evolution in cells grown under limited CO2 concentrations.