This experiment was designed to narrow the estimated range for lysine requirement of broiler chickens determined by isotopic techniques. In addition the influence of a long-term feed withdrawal previous 14C-lysine-injection on the lysine catabolism was investigated. 120 male broiler chickens 7 to 21 days posthatching received a diet based on wheat and wheat gluten. Lysine content was varied from 8.3 to 16.0 g/kg DM (3.2 to 6.3 g/16 g N) at 8 levels by supplementing the basal diet with L-lysine-HCl. After the feeding period animals of each group were labelled with 14C-L-lysine by intravenous injection 5.5 and 15.5 hours after feed withdrawal, respectively. During the following 4 hours the excretion of 14CO2 and CO2 was measured. Highest body weight gain was observed in the group with 13.8 g lysine/kg DM. In case of 14CO2 excretion measurements starting 5.5 hours after feed withdrawal an increase of 14CO2 excretion was observed if the lysine content of the diet exceeded 11.6 g/kg DM. This estimated range for lysine requirement (11.6 to 12.7 g/kg DM with 26% CP in the DM) was lower compared with the lysine requirement estimated by the growth curve (12.7 to 13.8 g/kg DM). This discrepancy could be explained by the fact that the results of the metabolism oriented determination of lysine requirement represent the requirement at the actual age, while the feeding experiment reflects a mean lysine requirement of the previous period of 14 days. If the animals were labelled with 14C-lysine 15.5 hours after feed withdrawal no clear response in 14CO2 excretion and specific radioactivity of CO2 on the dietary lysine content was observed.