The oxygen uptake in the light was investigated in the suspension of isolated pea thylakoids under the conditions of inhibition of electron transport from photosystem II by diuron and delivery of electrons to photosystem I by means of artificial donors in the presence of ascorbate. Effects of ascorbate and electron donors on the reduction of the O2 molecules by the components of acceptor side of photosystem I was analyzed. It was shown that dichlorophenolindophenol (DPIP) cannot be used as the donor for photosystem I for the study of this process. N, N, N', N'-Tetramethyl-/?-phenylenediamine (TMPD) applied as a donor does not seem to affect directly the reaction of O2 reduction, since an increase in its concentration did not increase the rate of oxygen uptake in the light. In experiments with TMPD, an increase in the light intensity caused an increase of oxygen uptake rate; this fact was interpreted as a consequence of the increase in the apparent rate constant of the reaction of the O2 reduction by components of acceptor side of photosystem I.