The closed greenhouse is a new development in protected cultivation. As air is not ventilated to the outside environment, CO2-concentrations are higher than in conventional greenhouses, causing 15¿20% increased production. The question remained as to whether increased air flow rates in a closed greenhouse cause photosynthetic adaptation. This was evaluated in a tomato crop that was planted in the summer of 2005 in The Netherlands in regular greenhouse compartments, in which air tubes were placed low and high in the canopy. Photosynthesis light-response curves were established at these two heights at 400, 700 and 1000 ppm of air CO2. Increased CO2 concentration and a higher position in the canopy caused an increase of the maximum photosynthetic rate, confirming earlier knowledge. However, the pattern of air circulation did not change the photosynthesis light-response curve. This corresponded with the absence of differences in total dry matter production and cumulative fruit growth. The INTKAM tomato model adequately simulated growth and development. It is therefore concluded that the pattern of air circulation did not cause adaptation of the photosynthetic apparatus, and that yield increases are attributable to the instantaneous effects of elevated CO2-concentrations.