The electron transfer from the photosynthetic system in cyanobacteria, Synechococcus sp. PPC7942 to exogenous electron accepters was examined using several electrochemical techniques. 1,4-Benzoquinone (BQ) and 2,6-dimethyl-1,4-benzoquinone (DMBQ) were found to function as good exogenous electron accepters for the photosystem. Kinetic analysis with rotating disk amperometry revealed that the photoreduction of these quinones proceeds in Michaelis-Menten type kinetics for the concentration of the quinones and the light intensity. The electron transfer rate of the BQ reduction was as high as 68% compared with that of the photosynthetic oxygen evolution. Synechococcus sp. cell-entrapped and DMBQ-embedded carbon paste electrodes provided steady-state current ascribed to the photoelectrochemical oxidation of water. Although several inhibitors against the photosynthetic system suppressed the photoelectrochemical response, phenylmercury acetate, which inhibits ferredoxin and ferredoxin-NADP oxidoreductase, was found to enhance the photocurrent. Some electrochemical aspects of this system are discussed.