In intact, uncoupled type B chloroplasts from spinach, added ATP causes a slow light-induced decline ( t 1 2 ≈ 3 min ) of chlorophyll a fluorescence at room temperature. Fluorescence spectra were recorded after fast cooling to 77 K and normalized with fluorescein as an internal standard. Related to the fluorescence quenching at room temperature, an increase in Photosystem (PS) I fluorescence ( F 735) and a decrease in PS II fluorescence ( F 695) were observed in the low-temperature spectra. The change in the F 735 F 695 ratio was abolished by the presence of methyl viologen. Fluorescence induction at 77 K of chloroplasts frozen in the quenched state showed lowered variable ( F v) and initial ( F 0) fluorescence at 690 nm and an increase in F 0 at 735 nm. The results are interpreted as indicating an ATP-dependent change of the initial distribution of excitation energy in favor of PS I, which is controlled by the redox state of the electron-transport chain and, according to current theories, is caused by phosphorylation of the light-harvesting complex.