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Responses of the tomato (Lycopersicon esculentum Mill. cv. Virosa) plant to elevated CO2 concentrations applied throughout the photoperiod or part of it were studied under two temperature regimes. Plants were exposed to CO2 at 340 (control), 700, and 1000 μl·liter–1. The highest concentration was applied only at 22/16C (day/night) and 700 μl·liter–1 at 22/16C and 25/16C. Transpiration rates were lower and photosynthetic rates were higher under elevated CO2 than at the ambient level. Biomass production was higher only for plants grown at 700 μl·liter–1 and 25/16C. Concentrations of macronutrients were lower in plants exposed to 1000 μl CO2/liter than in the control plants. Intermittent CO2 was applied using two timing methods. In method 1, plants were exposed to 4- or 8-hour high-CO2 concentrations during their 12-hour photoperiod. In method 2, plants were exposed for 3.5 days of each week to 700 μl CO2/liter. Only two of the 8-hour exposures resulted in greater growth than the controls. The lack of higher growth for CO2-enriched plants at 22/16C was attributed to a higher dark respiration rate and to a lack of efficient transport of photosynthates out of leaves.