Unsaturated lipids in Cucumis leaf discs at 1 °C undergo a photo-oxidative degradation which can be measured with the thiobarbituric-acid test. Kinetics of thiobarbituric-acid-reactivity (TBAR) show a lag phase followed by a rapidly increasing phase and a final decreasing phase. DCMU inhibits the photo-oxidative increase of TBAR. Among all combined fatty acids of Cucumis leaf discs linolenic acid is the most abundant (72.6%). The fatty acid composition of Cucumis leaf discs shows little alteration after 8 hours light at 1 °C but after 24 and 48 hours light there is a fast degradation of linolenic acid as compared to the rest of the fatty acids. A crude action spectrum shows a high maximum of TBAR in the blue and a lower maximum in the red spectral region and minimal TBAR in the green and in the far red region. It is suggested that chlorophyll and carotenoids are sensitizers of unsaturated fatty acid photo-oxidation. Possible connexions between chlorophyll and photooxidative fatty acid degradation are discussed.