A brief exposure (15-30 min) of green alga Chlamydomonas reinhardtii cells to high-intensity light (2000 mu mol m(-2) s(-1)) was accompanied by rapid suppression of photosynthetic O-2 evolution. The decline in the rate of O-2 evolution was followed by stimulation of H-2 production under light of 15 mu mol m(-2) s(-1). The effect was dependent on light intensity and cell suspension density. It appears that photoinhibition of photosynthetic O-2 evolution led to anaerobiosis that is favorable for H-2 production. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.