Four individual maturated Norway spruce trees (Picea abies) were exposed to elevated (ambient + 350 mmol(CO2) mol-1) CO2 concentration for five years using the open-top chambers (OTCs). The depression of CO2 assimilation was evident from the significant changes in both primary and secondary photosynthetic reactions, and subsequently from the level of biomass allocation. Short-term exposure to elevated CO2 was manifested by the increase of the CO2 assimilation because of suppressed fotorespiration and the increased carboxylation activity of RuBISCO enzyme. Statistically significant depression of the CO2 uptake and carboxylation efficiency after the long-term exposure to elevated CO2 was estimated from the combination of gas exchange and fluorescence measurements, and the application of a biochemical model of photosynthesis. The analysis shows that the depression of photosynthetic activity by long-term impact of elevated CO2 is mainly caused by decreased RuBISCO carboxylation rate. The modifications of photosynthetic assimilation depend on the time during the growing season. Data sets obtained during three subsequent growing seasons show a seasonal periodicity of downward regulation of photosynthesis caused by a deficiency of nitrogen (in spring) and by lack of active sinks for increased production of assimilates (in autumn).