The active transport of CO(2) in the cyanobacterium Synechococcus UTEX 625 was inhibited by H(2)S. Treatment of the cells with up to 150 micromolar H(2)S + HS(-) at pH 8.0 had little effect on Na(+)-dependent HCO(3) (-) transport or photosynthetic O(2) evolution, but CO(2) transport was inhibited by more than 90%. CO(2) transport was restored when H(2)S was removed by flushing with N(2). At constant total H(2)S + HS(-) concentrations, inhibition of CO(2) transport increased as the ratio of H(2)S to HS(-) increased, suggesting a direct role for H(2)S in the inhibitory process. Hydrogen sulfide does not appear to serve as a substrate for transport. In the presence of H(2)S and Na(+) -dependent HCO(3) (-) transport, the extracellular CO(2) concentration rose considerably above its equilibrium level, but was maintained far below its equilibrium level in the absence of H(2)S. The inhibition of CO(2) transport, therefore, revealed an ongoing leakage from the cells of CO(2) which was derived from the intracellular dehydration of HCO(3) (-) which itself had been recently transported into the cells. Normally, leaked CO(2) is efficiently transported back into the cell by the CO(2) transport system, thus maintaining the extracellular CO(2) concentration near zero. It is suggested that CO(2) transport not only serves as a primary means of inorganic carbon acquisition for photosynthesis but also serves as a means of recovering CO(2) lost from the cell. A schematic model describing the relationship between the CO(2) and HCO(3) (-) transport systems is presented.