In sulfur-deprived Chlamydomonas reinhardtii cells the activity of photosystem 2 (PSI!) has been shown to have a crucial role in the photosynthetic production of H-2, since it allows the synthesis of internal reserves such as starch. In the present investigation, the PSI! inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) was added in starch-enriched and sulfur-depleted C. reinhardtii cultures 1) at the very end of the aerobic phase, and 2) soon after the culture started to evolve H-2. In the former case, production of H-2 on a volumetric basis was completely down-regulated, although starch mobilization was enhanced. In vitro tests showed that the hydrogenase enzyme was active, although its efficiency of utilization in vivo was lowered very soon in the experiment. When an inhibitor of Rubisco such as glycolaldehyde was added under the same conditions, no substantial improvement in H-2 production rates was noted. These findings indicate that, aside starch storage, PSII plays an active role in the induction of the H-2 production process. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.