Diurnal variation in the chilling sensitivity of `Rutgers' tomato (Lycopersicon esculentum Mill.) seedlings was examined. Chilling sensitivity was highest in seedlings chilled at the end of the dark period, and these seedlings became more resistant to chilling injury on exposure to the light. The development of chilling tolerance in tomato seedlings was a response to light and not under the control of a circadian rhythm. The recovery of leaf gas exchange following chilling was faster in seedlings chilled at the end of the light period. Diurnal variation in chilling sensitivity was associated with changes in catalase and superoxide dismutase activities. An increase in catalase and superoxide dismutase activities was observed at the end of the light period. Catalase activity was significantly higher in all stages of chilling following the light period compared to those chilled after the end of the dark period. Forty-eight hours of 14 °C acclimation or pretreatment with hydrogen peroxide conferred increased chilling tolerance to tomato seedlings. Hydrogen peroxide-treated seedlings showed little evidence of a diurnal variation in chilling sensitivity. These results support a role for light and oxidative stress in conferring increased chilling tolerance to tomato seedlings.