Abstract Changes in total serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were investigated by HPLC EC in Ischnura graellsii over a larval-adult period including both sexes. Variationsof tryptophan (Trp) and protein levels were also studied. Significant higher levels of 5-HT were found in head portion for both sexes over the period studied, with mean values (in ng/mg protein) of 12.19–27.46 (head) and 2.63–6.06 (body) for males, and mean values of 3.13–26.48 (head) and 1.84–4.03 (body) for females. 5-HIAA concentration was also significantly higher in head portion for both sexes; mean values (in ng/mg protein) of 4.45–24.19 (head) and 0.87–8.63 (body) were observed for males, and mean values of 0.76–16.87 (head) and 1.18–7.12 (body) for females. The levels of the four compounds studied were significantly affected by the factor Instar. It is noticeable that significant differences between sexes for head Trp, 5-HT and 5-HIAA were observed in the last larval instar (F0), males showing higher values of 5-HT and 5-HIAA, and females higher values of Trp. In all cases, 5-HIAA/5-HT ratio decreased down to F0 and increased in adult stage. A positive correlation between 5-HT and 5-HIAA in both males ( P < 0.0005) and females ( P < 0.0001) was also observed. Our data suggest that oxidative deamination of serotonin in Ischnura may have higher importance than in other insects.