An electrometrical technique was used to investigate electron transfer between the terminal iron-sulfur centers F(A)/F(B) and external electron acceptors in photosystem I (PS I) complexes from the cyanobacterium Synechococcus sp. PCC 6301 and from spinach. The increase of the relative contribution of the slow components of the membrane potential decay kinetics in the presence of both native (ferredoxin, flavodoxin) and artificial (methyl viologen) electron acceptors indicate the effective interaction between the terminal 14Fe-4S] cluster and acceptors. The finding that FA fails to donate electrons to flavodoxin in F(B)-less (HgCl2-treated) PS I complexes suggests that F(B) is the direct electron donor to flavodoxin. The lack of additional electrogenicity under conditions of effective electron transfer from the F(B) redox center to soluble acceptors indicates that this reaction is electrically silent.