Serotonin (5-HT) is an important regulator of morphogenetic activities during early central nervous system development, including cell proliferation, migration, and differentiation. The 5-HT transporter (5-HTT) plays a pivotal role in brain 5-HT homeostasis. It is also the initial target for both antidepressant drugs and drugs of abuse, some of which are potent neurotoxins. A polymorphism in the 5'-flanking regulatory region of the 5-HTT gene that results in allelic variation of 5-HTT expression is associated with anxiety-related personality traits and may influence the risk of developing affective disorders. Progress in 5-HTT gene inactivation studies are also changing views of the relevance of adaptive 5-HT uptake function in brain development and plasticity as well as processes underlying drug dependence and neurodegeneration. Despite evidence for a potential role of the 5-HTT in the integration of synaptic connections in the mammalian brain during development, adult life, and old age, detailed knowledge of the molecular mechanisms involved in these fine-tuning processes is just beginning to emerge. Integration of various strategies, including molecular genetic, transgenic, and gene transfer techniques, will allow elucidation of the 5-HTT's role in brain development, plasticity, and degeneration as well as in affective illness, drug abuse, and dementia.