Diurnal and seasonal fluctuation in ambient CO2 concentration (C^) of more than 100 cm^ m‘3 in urban areas is an important abiotic factor influencing photosynthetic activity. This was shown in a young poplar tree under controlled environmental conditions. Here, the short-term rise in from 320 to 360 cm^ m‘3 during a day resulted in a 10 % higher CO2 gain. This variation in also influenced the interpretation of CO2 exchange data. Calculation of the intemal CO2 concentration in a leaf, Q, on the basis of an assumed mean value of atmospheric CO2 of 340 cm^ m'^ led to a possible 8 % divergence ffom the actual Cj in our experiments and thus also to a falše interpretation of the results, as it concealed the photosynthetic response to a rise or decline of Cg. Additionally, the changing CO2 concentration directly influenced the measurement by the infrared gas analyzer, because of its nonlinear response. Lack of compensation for this can lead to an additional inaccuracy of 3 to 8 % in the calculation of net photosynthetic rate and Q.