Tryptophan hydroxylase 2 (TPH2) is the rate limiting enzyme of serotonin synthesis in the brain. A recently described functional (C1473G) single nucleotide polymorphism in mouse TPH2 resulting in vitro in a strongly decreased enzymatic activity was suspected to be responsible for the observed differences in 5-HT levels and behaviour between mice strains. We bred two substrains of C57BL/6 mice carrying the two isoforms and could show that both exhibit equal TPH activity, brain 5-HT content and behaviour. These data indicate that the distinct behavioural characteristics of mouse strains are not due to differences in TPH2 activity, but to other variations in the genetic background.