The relative contribution of the two tryptophan hydroxylase (TPH) isoforms, TPH1 and TPH2, to brain serotonergic system function is controversial. To investigate the respective role of TPH2 in neuron serotonin (5-HT) synthesis and the role of 5-HT in brain development, mice with a targeted disruption of Tph2 were generated. The preliminary results indicate that in Tph2 knockout mice raphe neurons are completely devoid of 5-HT, whereas no obvious alteration in morphology and fiber distribution are observed. The findings confirm the exclusive specificity of Tph2 in brain 5-HT synthesis and suggest that Tph2-synthesized 5-HT is not required for serotonergic neuron formation.