Photosynthesis was measured in excised cucumber (Cucumis sativus L. cv. Marketer) cotyledons subjected to chilling temperatures in the presence or absence of photo synthetically active radiation (PAR), under conditions that precluded water stress. Cotyledon CO2 exchange rates (CER) were reduced after chilling in the light, but not after chilling in the dark. In the light, reduction occurred more rapidly at lower than at higher chilling temperatures. The level of PAR had little or no effect on CER reductions at lower temperatures within the chilling range, but at higher chilling temperatures, reduction occurred more rapidly at higher PAR levels. Recovery of photosynthesis following chilling in the light occurred more rapidly in the dark than in the light, and this difference in recovery rate was greater with longer exposure to light and chilling conditions. The removal of light during the recovery phase accelerated the rate of recovery to a level comparable to the dark control.