Non-photochemical quenching (NPQ) of Chl fluorescence is a mechanism for dissipating excess photon energy and is dependent on the formation of a DeltapH across the thylakoid membranes. The role of cyclic electron flow around photosystem I (PSI) (CEF-PSI) in the formation of this DeltapH was elucidated by studying the relationships between O sub(2)-evolution rate V(O sub(2)), quantum yield of both PSII and PSI Phi(PSII) and Phi(PSI), and Chl fluorescence parameters measured simultaneously in intact leaves of tobacco plants in CO sub (2)-saturated air. Although increases in light intensity raised V(O sub (2)) and the relative electron fluxes through both PSII and PSI Phi(PSII)x PFD and Phi(PSI)x PFD only Phi(PSI)x PFD continued to increase after V(O sub (2)) and Phi(PSII)x PFD became light saturated. These results revealed the activity of an electron transport reaction in PSI not related to photosynthetic linear electron flow (LEF), namely CEF-PSI. NPQ of Chl fluorescence drastically increased after Phi(PSII)x PFD became light saturated and the values of NPQ correlated positively with the relative activity of CEF-PSI. At low temperatures, the light-saturation point of Phi(PSII)x PFD was lower than that of Phi(PSII)x PFD and NPQ was high. On the other hand, at high temperatures, the light-dependence curves of Phi(PSII)x PFD and Phi(PSI)x PFD corresponded completely and NPQ was not induced. These results indicate that limitation of LEF induced CEF-PSI, which, in turn, helped to dissipate excess photon energy by driving NPQ of Chl fluorescence.