The effect of light, during pretreatment at 1 °C, on the capacity of Cucumis leaf discs to reduce TTC1 was studied. Light decreases the capacity of the cells to reduce TTC by a photo-oxidative process. The kinetics show a lag phase followed by a rapid decrease. 6% oxygen as compared to 100% oxygen lengthens the lag phase but does not alter the rate of damage during the rapid phase. DCMU2 has an inhibitive effect. A crude action spectrum of the decrease in TTC reduction shows that the blue and the red region of the spectrum are most effective, while the green and the far red region are relatively ineffective. It is suggested that chlorophyll as well as the carotenoids absorb the light energy which causes the harmful effect on the mechanism of TTC reduction.