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The aim of this study was to clarify to what extent the effects of elevated dietary L-tryptophan (Trp) on aggressive behavior and stress responsiveness in rainbow trout are mediated by circulating melatonin and central serotonin (5-HT), respectively. Isolated rainbow trout were paired for I It a day for 7 days in order to create fish with experience of being dominant and subordinate. Following this week, the fish were tested for aggressive behavior using a resident-intruder test after which they were subjected to one of four treatments: (1) tryptophan, (2) the selective serotonin reuptake inhibitor (SSRI) citalopram, (3) melatonin, and (4) no treatment (controls). After 7 days of treatment, the fish were subjected to a second resident-intruder test. Trp-supplemented feed resulted in a suppression of aggressive behavior in fish with experience of being dominant. Moreover, fish fed Trp-supplemented feed, regardless of social experience, also displayed lower plasma cortisol levels than controls. These effects of elevated dietary Trp were closely mimicked by citalopram treatment, whereas exogenous melatonin had no effect on either aggressive behavior or plasma cortisol. Thus, the effect of elevated dietary Trp on aggressive behavior and stress responses does not appear to be mediated by melatonin even though elevated dietary intake of Trp resulted in an increase in plasma melatonin concentrations.