Addition of ferredoxin to isolated thylakoid membranes reconstitutes electron transport from water to NADP and to O(2) (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.