When chloroplasts are aged for 5 min at pH 9.6, or are exposed to uncouplers at pH 8.5-9.0, electron flow from water to Hill acceptors is inhibited. Both treatments induce rapid millisecond dark decay of delayed light emission. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea-sensitive electron transport through Photosystem II can be regenerated in both types of inhibited chloroplasts by the artificial electron donor, 1, 5-diphenylcarbohydrazide. Neither treatment inhibits electron flow through Photosystem I. Uncouplers at alkaline pH, when added in the light, are less effective in producing the inhibition than when added in the dark. These results are interpreted as indicating inhibition of the oxygen-evolving apparatus by alkaline intrathylakoid pH.