Incubation of greening barley leaves with cesium chloride (CsCl) results in photodynamic leaf lesions within 24 h due to an inactivation of uroporphyrinogen III decarboxylase, an enzyme of tetrapyrrole biosynthesis, and transient accumulation of uroporphyrin (ogen). To examine the mechanism of porphyrinogenesis, time kinetics of the accumulating tetrapyrrole intermediates uroporphyrin (ogen) and protochlorophyllide were performed with leaves which were cut from 7-day-old dark-grown barley seedlings and incubated in 15 mM CsCl or water under different light regimes. In the presence of CsCl chlorophyll and carotenoids accumulation was inhibited in the first 24 h of continuous light and the pigment content decreased dramatically during extended illumination. When CsCl=treated leaves were transferred to darkness, accumulated uroporphyrinogen was completely converted to protochlorophyllide. Low temperature fluorescence spectroscopy confirmed that uroporphyrinogen almost completely accumulated in the reduced form. The oxidised form, uroporphyrin, was detectable after 24 h of illumination. The photodynamic leaf lesions became visible at the same time. Protochlorophyllide synthesised from accumulated uroporphyrinogen III in dark incubated leaves had a fluorescence maximum at 635 nm which is indicative for its non-photoconvertible form. Re-illumination of the barley leaves resulted in a rapid degradation of proteins and pigments and an intense lipid peroxidation within less than two hours due to the photodestructive potential of non-metabolised protochlorophyllide.