Oxidation of photosystem I (PSI) donors under far-red light (FRL), slow re-reduction by stromal reductants and fast re-reduction in the dark subsequent to illumination by white light (WL) were recorded in leaves of several C(3) plants at 810 and 950 nm. During the re-reduction from stromal reductants the mutual interdependence of the two signals followed the theoretical relationship calculated assuming redox equilibrium between plastocyanin (PC) and P700, with the equilibrium constant of 40 +/- 10 (Delta E (m) = 86-99 mV) in most of the measured 24 leaves of nine plant species. The presence of non-oxidizable PC of up to 13% of the whole pool, indicating partial control of electron transport by PC diffusion, was transiently detected during a saturation pulse of white light superimposed on FRL or on low WL. Nevertheless, non-oxidizable PC was absent in the steady state during fast light-saturated photosynthesis. It is concluded that in leaves during steady state photosynthesis the electron transport rate is not critically limited by PC diffusion, but the high-potential electron carriers PC and P700 remain close to the redox equilibrium.