Ultraweak light emission (UWLE) was measured from leaves of various chillingsensitive (Lycopersicon esculentum, Cucumis sativus and Phaseolus vulgaris) and tolerant (Pisum sativum and Vicia faba) plants after exposure to low (47C) temperature in the light. UWLE increased upon chilling treatment combined with illumination with 200 mol m2 s1 PAR in all plants, by about 30 in tolerant and by more than 100 in sensitive plants. It increased more when applied together with 400 mol m2 s1 PAR: by 90100 and by 250280 in chillingtolerant and sensitive plants, respectively. Free radical production was detected by spintrapping EPR spectroscopy in thylakoid membranes isolated from the chillingtreated Lycopersicon esculentum and Vicia faba leaves. After 12 h chilling at 200 mol m2 s1 PAR, free radical production was approximately 3 times greater in the former than in the latter species. The same ratio was approximately 6 if chilling was carried out at 400 mol m2 s1 PAR, indicating the role of photooxidative stress in chilling injury.Our results also confirm that the stressinduced increase in UWLE is an indicator of free radical production and offers the possibility of using UWLE for monitoring the effect of chilling on temperaturesensitive plants in an early stage.