The neural transmitter serotonin (5-HT) and the serotinergic system are involved in several abnormal behaviour patterns in animals. In this study the authors examined the relationship between this system and the incidence of feather pecking in 7 week old chickens. 5-HT is derived from tryptophan and they increased serotonin levels by increasing dietary tryptophan. Chicks were fed either the control diet with 0.16% tryptophan, or the treatment diet with 2%. Two lines of laying birds were used. Tryptophan reduced the incidence of gentle feather pecking, but only a trend in the same direction was found for severe feather pecking. Physiological measures confirmed that the high levels of tryptophan increased blood levels of the amino acid and increased serotonin in the central nervous system. High tryptophan also resulted in an increase in cortisol levels in the blood, which has been reported as a physiological effect of the amino acid previously. High levels of dietary tryptophan was effective in reducing levels of feather pecking in young chicks.