The use of computer-assisted electron spin resonance in conjunction with oxidation-reduction potentiometry and inhibitor studies is directed toward understanding the intermediates and mechanisms associated with oxygen evolution in Photosystem 2 of green plant photosynthesis. Research efforts are being focussed on the methods of inhibition of Photosystem 2; the sites of inhibition as monitored by room temperature and cryogenic esr spectroscopy; the pathways of cyclic and noncyclic electron transport in Photosystem 2; the molecular environment of Manganese and the interaction of this divalent cation with other intermediates in oxygen evolution. In addition, we are continuing to characterize conditions favorable for reactivation of oxygen evolution in inhibited chloroplasts. To this end we have developed preliminary techniques that permit 100% reactivation of oxygen production in spinach chloroplasts totally inhibited by 0.2M magnesium. The parameters of oxygen evolution in the restored samples are being characterized by esr spectroscopy, manganese analysis, electron transport studies and steady-state and flash oxygen measurements.