It is shown that the light-dependent proton uptake by a suspension of isolated chloroplasts is completely inhibited in the presence of 100 - 200 <$E mu>M <$E roman Zn sup 2+> ions at the <$E 5~roman {Zn sup 2+ "/" Chl}> ratio. At the same time, in the presence of 30 - 200 <$E mu>M ZnSO4, the rate of photosynthetic oxygen uptake reduced no more than by 20 - 30 % from control and up to 50 % <$E DELTA> pH control value remained. The results allow us to suppose that, in the presence of zinc ions, electron transport in PS2 is inhibited at the level of secondary quinone acceptor, QB, photoreduction of which is accompanied by proton uptake from external medium; an alternative pathway of electron transfer to terminal acceptor is activated providing the water photooxidation and the transmembrane proton gradient formation.