Copper inhibited chlorophyll fluorescence intensity at room temperature in isolated Chinese cabbage chloroplasts, and copper treatment induced pigment degradation. Fluorescence inhibition and pigment degradation by copper treatment was higher at acidic condition than at alkaline condition. Light illumination after copper treatment increased the copper toxicity upon the pigment content and fluorescence intensity. The pH dependency of copper toxicity on fluorescence and pigment degradation was different from the pH dependency of that on PS-2 mediated O₂ evolution. These results suggest that copper inhibition on fluorescence intensity may be due to the conformational changes of thylakoids. Copper inhibited chlorophyll fluorescence intensity at room temperature in isolated Chinese cabbage chloroplasts, and copper treatment induced pigment degradation. Fluorescence inhibition and pigment degradation by copper treatment was higher at acidic condition than at alkaline condition. Light illumination after copper treatment increased the copper toxicity upon the pigment content and fluorescence intensity. The pH dependency of copper toxicity on fluorescence and pigment degradation was different from the pH dependency of that on PS-2 mediated O₂ evolution. These results suggest that copper inhibition on fluorescence intensity may be due to the conformational changes of thylakoids.