The cyanobacterium Synechocystis 6803 contains a bi-directional hydrogenase, encoded by the hox (E, F, G, Y, H) genes, which form an operon. Regulation of the hox genes is poorly understood, therefore, we applied quantitative RT PCR to follow changes in the mRNA level of hoxE in light and dark, as well as in the absence and presence of oxygen. We show that the expression level of hoxE is induced by anaerobiosis, in darkness, and also after inhibition of photosynthetic electron transport in the light at the level of the secondary quinone electron acceptor by DCMU. Addition of H2O2 prevents the increase of the hoxE transcript under the above conditions, and leads to the decrease of the transcript level induced by darkness or anaerobic conditions. Similiar effects were observed with the other members of the hox operon.Our data demonstrate that the regulation of the hoxE gene (and of the whole hox operon) is under light- and redox control, which leads to enhanced expression in the absence of photosynthetic electron transport through Photosystem II, and in anaerobiosis. This appears to be a crucial step in the regulation pathways which ensure that the oxygen sensitive hydrogenase enzyme is produced in the absence of oxygen.