If low night temperatures can be combined with high day temperatures, providing optimal growth conditions for plants, a significant energy saving can be achieved in greenhouses. Lowering the night temperature from 18°C to 10°C-11°C for 8 h had no negative effect on the CO₂ exchange rate (CER) during the following light period in tomato. This was found both in plants grown in artificial light only or in combination with daylight. Allowing the temperature to increase from 20°C to about 40°C, in parallel with an increasing solar photon flux density (PFD) from 0 up to about 800 μmol·m^-2·s^-1 in the greenhouse during summer, progressively increased CER when the CO₂ concentration was maintained at 900 μmol·mol^-1. At 400 μmol·mol^-1 CO₂, maximum CER was reached at about 600 μmol·m^-2·s^-1 PFD combined with a temperature of 32°C, and leveled out with a further increase in PFD and temperature. Maximum CER at high CO₂ concentration was around 100% higher than at low CO₂ level. Under early autumn conditions, CER increased up to about 500 μmol·m^-2·s^-1 PFD/32°C at low CO₂ and up to about 600 μmol·m^-2·s^-1 PFD/35°C at high CO₂. An elevated CO₂ level doubled the CER in this experiment as well. Measurements of chlorophyll fluorescence showed no effect of low night temperature, high day temperature or CO₂ concentration on the quantum yield of photosynthesis, indicating that no treatment negatively affected the efficiency of the photosynthetic apparatus. The results showed that low night temperatures may be combined with very high day temperatures without any loss of daily photosynthesis particularly in a CO₂ enriched atmosphere. If this can be combined with normal plant development and no negative effects on the yield, significant energy savings can be achieved in greenhouses.