We investigated the photodynamic action of hypericin, a natural naphthodianthrone, on photosynthetic electron transport and fluorescence of the cyanobacterium Anacystis nidulans (Synechococcus 6301). The most drastic effect was the inactivation of photosynthetic oxygen evolution in the presence of the electron acceptor phenyl-pbenzoquinone in aerobic cells which required 1 hypericird5 chlorophyll a for half-maximal effect. Anaerobic A. nidulans was only partially inactivated and variable chlorophyll a fluorescence remained unperturbed suggesting that photoreaction center II was not a target. Further, hypericin, stimulated photoinduced oxygen uptake in the presence of methylviologen in aerobic cells. This action was less specific than the inactivation of oxygen evolution (1 hypericin/0.5-0.7 chlorophyll a for half-maximal effect). Results point to the involvement of molecular oxygen in two ways. Type I mechanism (Henderson BW and Dougherty TJ (1992) Photochem Photobiol 55: 145-157) in which ground state oxygen reacts with excited substrate triplets appears probable for the inactivation of oxygen evolution. On the other hand, Type II mechanism in which excited oxygen singlets react with ground state substrate molecules appears probable in the stimulation of methylviologen mediated oxygen uptake.