A method was developed to quantify the fraction of photosystem I (PSI) centers that operate according to the cyclic or linear mode, respectively. P(700) and plastocyanin oxidation were analyzed under a weak far-red excitation (approximately eight photons per s(-1) per PSI) that induces P(700) oxidation in approximately 20 s and approximately 3 s in dark-adapted and preilluminated leaves, respectively. This finding implies that, in dark-adapted leaves, most of the electrons formed on the stromal side of PSI are transferred back to PSI through an efficient cyclic chain, whereas in preilluminated leaves, electrons are transferred to NADP and then to the Benson-Calvin cycle. Preillumination thus induces a transition from the cyclic to the linear mode. A reverse transition occurs in the dark in a time that increases with the duration and intensity of preillumination. After a approximately 10-min illumination under strong light that activates the Benson-Calvin cycle, the transition from the linear to the cyclic mode is completed in >1 h (t(1/2) approximately 30 min). The fraction of PSI involved in the cyclic process in dark-adapted leaves can be close to 100%. An apparent equilibrium constant of approximately 4 between P(700) and plastocyanin was measured during the course of the far-red illumination. This value is much lower than that computed from the midpoint redox potential of the two carriers (approximately 30). These results are interpreted assuming that chloroplasts include isolated compartments defined on the basis of the structural organization of the photosynthetic chain proposed by Albertsson [Albertsson, P. A. (2001) Trends Plant Sci. 6, 349-354].