None of the PS I-reducible molecules (nitrate, NAD or NADP) decreased H-2 evolution in Oscillatoria chalybea; indicating that their reduction would not compete with proton reduction. Seemingly, CO2 reduction does not compete as well. Only nitrite was accompanied with a strong and consistent inhibition most probably due to;a specific characteristic of the chemical rather than substrate competition. Such nitrite-induced inhibition could not be attributed to a lack in photosynthetic electrons since nitrite inhibited neither PS I nor PS II activity. ATP enhanced hydrogen evolution (about 50%) while NH4Cl in an uncoupling concentration was inhibitory; although in hydrogenases (other than nitrogenases) ATP is not directly involved (discussed). (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.