A continuous dual13CO2and15NH415NO3labelling experimental set-up is presented that was used to investigate the C and N uptake and allocation within 3-year old beech (Fagus sylvaticaL.) during one growing season. The C and N allocation pattern was determined after six, twelve and eighteen weeks of growth. The carbon uptake was distinctly different in the three phases examined: The first six weeks after budbreak were dedicated to leaf growth with aR/S(root to shoot) ratio of 0.14 for the new carbon. The second growth phase showed a balancedR/Sratio of C allocation and after week 13, the root compartment was the main carbon sink (R/S= 6.97).