Root respiration consumes an important part of the daily assimilated carbon but the magnitude of this component of forest net ecosystem exchange and its partitioning among the different energy demanding processes in roots are still poorly documented. 5-month old Eucalyptus cuttings were grown in a greenhouse in pot filled with coarse sand. They were fertilized with three different amounts of a slowrelease fertilizer with the doses of 8, 24 and 48 g of nitrogen per plant. Root respiration was measured using an infrared gas analyser by perfusing air through the pot on 9 plants per treatment on three dates 14 days apart. Measure of root respiration of the three treatments over time was made in order to obtain a large range of growth and nutrient uptake. Root respiration normalized at 22°C ranged from 0.09 to 0.23 gC d?1 for the three treatments during all the experiment. It was well predicted with a model that includes root growth rate and root nitrogen content. The nitrogen related maintenance coefficient was negatively correlated to the root nitrogen concentration suggesting a decrease in protein turnover with increasing fertility. Growth rate of fine root in a virtual stand was simulated using age-related allometric equations and further used to estimate root respiration in the field. Simulated root respiration increased over time from 0.39 to 3.14 gC m?2 d?1 between 6 and 126 months assuming a turnover of 2 yr?1 for fine roots. The major fraction of simulated root respiration in the field (78-92%) was used for the maintenance of the existing biomass.