The aim of this study was to investigate the effect of exposure to L-tryptophan (TRP) on the metabolism of 5-hydroxytryptamine (5HT) and behavior of medaka. In the first experiment, the fish were exposed to a 0, 1, 2 or 4 g/l of TRP solution for 24 hr. Although no significant difference in the brain 5HT content was detected, 5-hydroxyindoleacetic acid (5HIAA), a major 5HT metabolite, increased in a dose-dependent fashion. In the second experiment, the fish were maintained in a 0 or 4 g/l of TRP solution for 28 hr, and then their behaviors were monitored. The fish reared in under TRP solution were divided into two groups and transferred to either fresh water or a TRP solution. The locomotion of the TRP-treated group significantly increased compared to the control group irrespective of water conditions. It was suggested that TRP exposure activated the brain 5HTnergic systems and stimulated behavior of medaka.