Addition of ferredoxin to isolated thylakoid membranes reconstitutes electron transport from water to NADP and to 02 (the Mehler reaction).This electron flow is coupled to ATP-synthesis, and both cyclic and noncyclic electron transport drive photophosphorylation. Under conditions where the NADPH/NADP+ ratio is varied, the amount of ATP synthesis due to cyclic activity is also varied, as is the amount of cyclic activity which is sensitive to antimycin A. Partial inhibition of photosystem II activity with DCMU (which affects reduction of electron carriers of the interphotosystem chain) also affects the level of cyclic activity. The results of these experiments indicate that two modes of cyclic electron transfer activity, which differ in their antimycin A sensitivity, can operate in the thylakoid membrane. Regulation of these activities can occur at the level of ferredoxin and is governed by the NADPH/NADP ratio.Evidence has accumulated for the existence of PSI cyclic photophosphorylation activity and its importance for CO2 fixation in chloroplasts (6,7,15,24,25). One of the remaining questions about PSI cyclic electron flow pertains to the mechanism by which the chloroplast regulates cyclic and noncyclic electron transport in order to maintain the necessary balance between ATP and NADPH for CO2 fixation activity. Partial inhibition of PSII electron flow has been shown to stimulate cyclic photophosphorylation in intact chloroplasts ( 11,12,15) and in isolated thylakoids supplied with ferredoxin (3) as long as anaerobic conditions are maintained. This 'poising' effect presumably reflects a redistribution of electron flow in favor of reduced ferredoxin to PQ3 (with possible intermediate carriers) concurrent with restriction of electron flow from PSII to PQ. Addition of NADPH to isolated thylakoids in the presence of ferredoxin also stimulates cyclic photophosphorylation (1), and the NADPH/NADP+ ratio has been proposed to regulate the partitioning of electrons between the cyclic and noncyclic pathways at the level of ferredoxin (1,2,16,17,21 cyclic photophosphorylation, catalyzed by electron transport from water to ferredoxin/02 or to ferredoxin/NADP+. These experiments, performed under aerobic conditions using saturating light, provide a closer approximation of the activity of PSI cyclic photophosphorylation in vivo than does measurement of a cyclic reaction poised independently of noncyclic electron transport. Our previous studies on this reconstituted electron transport system have shown that electron transport to either ferredoxin/02 or to ferredoxin/NADP+ produces elevated P/O ratios (1.6 versus 1.25 with MeV as the acceptor); the increased P/O ratio is due to the induction, by ferredoxin, of a cyclic or Q-loop electron transfer mechanism (10). This work also suggests the existence of two separate pathways of cyclic photophosphorylation: one pathway is characterized by sensitivity to antimycin A and the requirement for a substantial pool of reduced ferredoxin, while a second pathway is insensitive to anti...