The light-induced changes in the yield of chlorophyll alpha fluorescence and photooxidation of P-700 in the P-700-enriched particles isolated from spinach chloroplasts were studied. 1. Fluorescence emitted from the particles was found to show light-induced transient changes in the yield. In the presence of ascorbate, illumination induced quenching of fluorescence in parallel to the photooxidation of P-700. The time course of dark reduction of photooxidized P-700 agreed well with that of dark recovery of variable fluorescence yield in the presence of ascorbate. When illuminated in the presence of dithionite, the emission yield increased, whereas no photooxidation of P-700 was observed. 2. The yield of variable fluorescence and redox state of P-700 depended similarly upon the redox potential. 3. At liquid nitrogen temperature, illumination induced a rise of the fluorescence yield and a complete photooxidation of P-700 in the ascorbate-treated sample. When the particles had been preincubated with dithionite in the light before cooling, light-induced rise in the fluorescence yield was accompanied by only a small extent of P-700 photooxidation. It is suggested that both the oxidized form of P-700 and the primary electron acceptor act as quenchers for the variable fluorescence. 4. The emission spectrum for the constant part of fluorescence (F679) has a peak at 679 nm, and that for the variable part of fluorescence (F694) has a peak at 694 nm at room temperature. The emission maxima were slightly shifted and the yield of variable fluorescence was markedly enhanced at liquid nitrogen temperature. 5. Excitation spectra determined show a peak at 672 nm for F679, and a peak at 672 nm and a shoulder at 685 nm for F694. Action spectrum for P-700 photooxidation was similar to the excitation spectrum for F694.