Total in vitro activity of RuBPCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) enzyme was assayed spectrophotometrically by the continuous measurement of 3-phosphoglycerate-dependent NADH oxidation in a coupled enzyme system. RuBPCO activities were found in the ranges 1.01-2.76 and 1.23-3.10 µmol(CO2) m-2 s- 1 in current Norway spruce needles growing in ambient (AC) and elevated (EC) CO2 concentration, respectively. RuBPCO activity in AC needles from the upper layer (U) was 11-15 % higher compared to those from the middle (M) layer, and even 44-56 % higher compared to the lower (L) layer of spruce crown. Over the vegetation season, we observed a highly significant decrease of RuBPCO activity in the EC-U needles from 3.10 (July) to 1.60 (October) µmol(CO2) m-2 s-1 as a consequence of downward feedback regulation. Moreover, this down-regulation was not caused by a non-specific decrease in total leaf nitrogen content. and M. Hrstka, O. Urban, M. V. Marek.