The possible protection against photo-oxidative degradation of chlorophyll and carotene in Cucumis leaf discs at 1 °C by a wide variety of compounds was investigated. Dimethyl sulphoxide and glycerol as well as ferricyanide, ascorbate, KCN and hydroxylamine were ineffective. Photo-oxidation was, however, at least partly, prevented by benzoquinone, hydroquinone, naphtoquinone, guajacol, benzidine, diaminobenzidine, alizarinesulphonicacid, triphenyltetrazoliumchloride and 3-(3,4-dichlorophenyl), 1,1-dimethylureea. Photooxidative damage was promoted by salicylaldoxine, disalicylidenediaminopropane, azide, ethylenediamine tetraacetic acid, trishydroxymethylaminomethane, diquat and benzylviologen. It was also increased at high pH values. It is suggested that the mechanism of protection of Cucumis leaf chloroplasts against low temperature photo-oxidative damage is a protection of the electron transport pathway from the reducing to the oxidizing side of the photosystems.