Photoinhibition of Photosystem I at chilling temperatures was investigated. Illumination of barley and cucumber leaves at 4°C induced a lowered Photosystem I activity. In barley, the reaction centre proteins PSI-A and PSI-B were both partially degraded as was the nuclear-encoded PSI-D polypeptide. Barley leaves infiltrated with KCN to increase oxidative stress, showed increased photoinhibition of Photosystem I, including reduced photochemical activity and marked degradation of several Photosystem I polypeptides. The most rapid and pronounced degradation was found in the PSI-D and PSI-E polypeptides exposed at the Photosystem I acceptor side. The PSI-A, -B, -C, -G, -H, -K and -L polypeptides were less extensively damaged. No damage of the lumenally oriented PSI-F and -N polypeptides was detected. The elevated photoinhibition of Photosystem I seen in KCN treated barley is most likely induced by a combination of increased active oxygen due to inhibited scavenging and increased accumulation of reducing power due to inhibition of the Calvin cycle. In barley, photo-inactivation of Photosystem I closely followed the degradation of PSI-A and PSI-B. Illumination of cucumber resulted in a pronounced loss of activity and appearance of specific PSI-A and PSI-B degradation products whereas the total PSI-A/B degradation was small. The PSI-A/B degradation identified in barley is interpreted to reflect a physiologically relevant process being part of a repair cycle, whereas the much smaller PSI-A/B degradation observed in cucumber is interpreted to represent an irreversible damage induced far below the temperature tolerance for cucumber.