This manuscript reviews the literature on hydrogen metabolism in blue-green algae and reports some new data from this laboratory. H2-formation by intact cells is found to be catalyzed exclusively by nitrogenase. Its rate appears to be variable from strain to strain used byt is--in our hands--very small. Therefore, blue-green algae are presumably of limited value in projects of solar energy conversion to form molecular hydrogen. These organisms are also able to consume the gas in a reaction catalysed by hydrogenase. Hydrogen is mainly consumed in an oxygen dependent reaction, as in aerobic nitrogen fixing bacteria. It can also serve as an electron donor for nitrogen fixation under certain physiological conditions. In experiments with a cell-free preparation, hydrogenase is found to be membrane-bound. The enzyme is characterized with respect to its specifity towards electron donors and acceptors.