Cesium chloride treatment of illuminated barley leaves leads to accumulation of uroporphyrinogen which is subsequently either oxidised to uroporphyrin in continuous light or converted to protochlorophyllide in darkness [Shalygo et al. (1998) J Photochem Photobiol 42: 151-158]. We were interested to elucidate the differences in the phototoxicity of uroporphyrin and protochlorophyllide in the CsCI-treated leaves. Photosensitization and the induction of oxidative stress responses in the barley leaves occurred much faster upon protochlorophyllide than upon uroporphyrin accumulation. We compared the time resolved changes in the pool sizes of low molecular weight antioxidants, such as ascorbate, glutathione and tocopherol, as well as of the enzymatic activities of catalase, ascorbate peroxidase, glutathione reductase and superoxide dismutase in illuminated barley leaves which accumulated uroporphyrin or protochlorophyllide. A rapid loss of the antioxidant levels correlated with the accumulation of reactive oxygen species. The contents of low molecular weight antioxidants and the activities of most of the antioxidative enzymes declined more rapidly in the presence of protochlorophyllide than of uroporphyrin. Due to its high lipophilicity, free protochlorophyllide is associated with biomembranes. Therefore, it is assumed that it exerts its phototoxic effects to membranes more rapidly than uroporphyrin.