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Hydrogen metabolism in a non-nitrogen-fixing cyanobacterium, Microcystis aeruginosa, was studied. The cyanobacterium evolved hydrogen gas under dark and anaerobic conditions. The rate of hydrogen evolution was higher in cells grown under nitrogen-limited conditions than under nitrogen-sufficient conditions. Hydrogen uptake occurred immediately when light was irradiated to the hydrogen-evolving cells in darkness. The light-dependent uptake of hydrogen was not sensitive to the inhibitor for photosystem II, DCMU [3-(3,4-dichloro-phenyl)-1,1-dimethyl urea], but sensitive to the antagonist of plastoquinone, DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone). These results suggest that hydrogen may be produced with degradation of endogenous storage materials, such as glycogen, and be taken up by the light-dependent reaction of photosystem I via plastoquinone.