We hypothesized that cyclic electron flow around photosystem I (CEF-PSI) participates in the induction of non-photochemical quenching (NPQ) of chlorophyll (Chl) fiuorescence when the rate of photosynthetic linear electron flow (LEF) is electron-acceptor limited. To test this hypothesis, the relationships among photosynthesis rate, electron fluxes through both PSI and PSII Je(PSI) and Je(PSII)l and Chl fluorescence parameters were analyzed simultaneously in intact leaves of tobacco plants at several light intensities and partial pressures of ambient C02 (Ca). At low light intensities, decreasing Ca lowered the photosynthesis rate, but Je(PSI) and Je(PSII) remained constant. Je(PSl) was larger than Je(PSII), indicating the existence of CEF-PSI. Increasing the light intensity enhanced photosynthesis and both Je(PSI) and Je (PSII). Je(PSI)/Je(PSII) also increased at high light and at high light and low Ca combined, showing a strong, positive relationship with NPQ of Chl fluorescence. These results indicated that CEF-PSI contributed to the dissipation of photon energy in excess of that consumed by photosynthesis by driving NPQ of Chl fiuorescence. The main physiological function of CEF-PSI in photosynthesis of higher plants is discussed.