Time courses of chlorophyll fluorescence at room temperature and fluorescence spectra at 77 K were measured to investigate the light-induced changes in the distribution of light energy between the two photosy stems in young spinach leaves. Illumination of the dark adapted leaves with primarily system II light induced typical fluorescence transients at room temperature. Fluorescence spectra at 77 K showed that the intensity of system II fluorescence at 77 K changed nearly in parallel with the fluorescence transients at room temperature within the range from M1 to T during illumination of the leaf.Illumination of the dark adapted leaves with light I produced an increase of system II fluorescence measured at 77 K. The characteristics of the changes induced by light I or II were different, showing that these two effects are related to different mechanisms.These results suggest that the dark state in spinach leaves is state II, that light I induces a state II to I transition, while light II induces fluorescence changes that are produced by mechanisms other than state I-state II transitions.