Exposure of tomato plants (Lycopersicon esculentum Mill. cv. Floramerica) to chilling temperatures in the dark for as little as 12 h resulted in a sizable inhibition in the rate of light- and CO2-saturated photosynthesis. However, when photosynthesis was measured at low light intensity, the inhibition disappeared and the quantum yield of CO2 reduction was diminished only slightly. Chilling the tomato plants under strong illumination caused an even more rapid and severe decline in the rate of light- and CO2-saturated photosynthesis, accompanied by a large decline in the quantum efficiency. Sizeable inhibition of photosystem II activity was observed only after dark exposures to low temperature of grater than 16 h. No inhibition of photosystem I electron transfer capacity was observed even after 40 h of dark chilling. Chilling under high light resulted in a rapid decline in both photosystem I and photosystem II electron transfer capacity as well as in significant reaction center inactivation.Regardless of whether the chilling exposure was in the presence or absence of illumination and regardless of its duration, the electron transfer capacity of thylakoid membranes isolated from the treated plants was always in excess of that necessary to support light- and CO2-saturated photosynthesis. Thus, in neither case of chilling inhibition of photosynthesis does it appear that impaired electron transfer capacity represents a significant rate limitation to whole plant photosynthesis.