The phytotoxic effect of Cu via the photosynthetic electron transport system was studied with isolated spinach chloroplasts. Cu(II) ions induce a light-driven peroxidation of membrane lipids leading to ethylene formation, the latter dominating over a concurrent ethane production. Seemingly, the hydroxyl radical originating from superoxide anion is the starting reactive O(2) species. Cu ions inhibit photosynthetic electron transport and apparently catalyze the formation of hydroxyl radical and Fenton-type reactions that result in destruction of unsaturated membrane fatty acids. The concept on the mode of action of Cu(II) and Cu(I) ions in lipid peroxidation as presented here suggests the influence of Cu on different reactions. Two sites are in the photosynthetic redox system; Cu participates in two Fenton-type reactions and in the conversion of ethyl radical to ethylene and ethane.