Eucalyptus regnans F. Muell. is one of the most commercially important timber trees in Australia. Study on the relationship between fertilizer application, amino acid accumulation and transformation and insect herbivory for eucalypts is both economically and ecologically necessary. E. regnans seedlings were grown with two rates of phosphorus (100 and 200 kg hm-2) in two soils with different P fixation capacity. The seedlings were grown in a glasshouse with 4 replicates. Experiment results showed: 1) seedling growth was significantly affected by the different soils and P treatment; 2) N and P concentrations in plant components were not significantly different with the soil and P treatments, while N and P concentrations in leaves decreased with leaf age; 3) amino acid concentration of xylem sap was dominated by glutamine and significantly affected by P application (concentration in xylem sap of seedlings grown with 100 kg hm-2 was 2 times more than that with 200 kg hm-2); 4) amino acid composition and concentration in leaves significantly changed with leaf age, particularly for arginine which was several-fold greater in old leaves than in young leaves. The change of arginine concentration in leaves is most likely due to some protein degradation and in situ arginine synthesis because arginine is not generally phloem sap mobile. The fact that glutamine concentration in xylem sap was dominant and inversely related to seedling growth suggests that specific nitrogenous solutes may be useful indices of the nitrogen status of E. regnans for insect herbivory.