The rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase (TPH), was studied in brain areas of male mice with repeated experience of victory (winners) or defeat (losers) gained in 10 daily agonistic confrontations. A reduction of TPH activity in the midbrain and an increase in the hypothalamus was demonstrated for winners compared with controls. In contrast, repeated defeat in social confrontations was associated with higher TPH activity in the striatum and hypothalamus in losers compared with controls. Agonistic interactions did not affect TPH activity in the amygdala, nucleus accumbens or hippocampus in either winners or losers. The sensory contact technique used in this work for generating winners and losers may be productive in the analysis of TPH gene regulation.