An increase of medium NaCl concentration induces Dunaliella cells to evolve O(2) photosynthetically even in the absence of CO(2). This NaCl-induced O(2) evolution may reflect the induced conversion of reserve carbohydrate to glycerol. The quantum yield for the NaCl-induced O(2) evolution, in the absence of CO(2), is 1.5-fold higher than that obtained for CO(2) fixation. Since the synthesis of glycerol from reserve carbohydrate in the absence of CO(2) requires only 0.5 ATP/NADPH, whereas photosynthesis requires at least 1.3 ATP/NADPH, it is concluded that the ATP/2e(-) ratio coupled to NADP reduction in Dunaliella is lower than required for CO(2) fixation.