5-hydroxytryptophan (TRP) is the immediate precursor to the biosynthesis of the neurotransmitter 5-hydroxytryptamine (5-HT; serotonin, the precursor of melatonin). In fish, the 5-HT system is believed to stimulate the hypothalamus-pituitary-interrenal (HPI) axis leading to elevated concentrations of circulating adrenocorticotropin releasing hormone (ACTH) and cortisol. Thus, 5-HT plays a central role in the primary stress response, leading to a general suppression of behavioural responses to environmental stimuli. However, in stressed fish, the selective 5-HT1A receptor agonist 8-OH-DPAT has an inhibitory effect on the HPI axis. The 5-HT system seems to be remarkably well conserved among all vertebrates, and increased availability of TRP has been shown to prevent depression and increases in plasma cortisol in response to uncontrollable stress in human subjects. In a series of studies we aimed to reveal the effects of elevated dietary TRP on brain 5-HT synthesis, behaviour and neuroendocrine stress responses, and to explore if TRP supplemented feed can be used to improve fish performance and welfare in aquaculture. The results showed that increased availability of TRP in the brain suppressed aggressive behaviour and HPI axis reactivity in rainbow trout, probably mediated by the brain 5-HT system rather than increases in circulating melatonin. Further, the time-courses of the effects of TRP on aggressive behaviour and post stress plasma cortisol suggest effects on 5-HT receptor densities and receptor mechanisms rather than a direct effect of 5-HT synthesis and release. Further, these results indicate the usefulness of dietary TRP in aquaculture to reduce the effects of both social and non-social stressors associated with production of fish for food consumption.