The sodium-dependent, high affinity serotonin [5-hydroxytryptamine (5-HT)] transporter (5-HTT) provides the primary mechanism for inactivation of 5-HT after its release into the synaptic cleft. To further evaluate the function of the 5-HTT, the murine gene was disrupted by homologous recombination. Despite evidence that excess extracellular 5-HT during embryonic development, including that produced by drugs that inhibit the 5-HTT, may lead to severe craniofacial and cardiac malformations, no obvious developmental phenotype was observed in the 5-HTT^−/− mice. High affinity [³H]5-HT uptake was completely absent in 5-HTT^−/− mice, confirming a physiologically effective knockout of the 5-HTT gene. 5-HTT binding sites labeled with [¹²⁵I]3β-(4′-iodophenyl)tropan-2β-carboxylic acid methyl ester were reduced in a gene dose-dependent manner, with no demonstrable binding in 5-HTT^−/− mutants. In adult 5-HTT^−/− mice, marked reductions (60–80%) in 5-HT concentrations were measured in several brain regions. While (+)-amphetamine-induced hyperactivity did not differ across genotypes, the locomotor enhancing effects of (+)-3,4-methylenedioxymethamphetamine, a substituted amphetamine that releases 5-HT via a transporter-dependent mechanism, was completely absent in 5-HTT^−/− mutants. Together, these data suggest that the presence of a functional 5-HTT is essential for brain 5-HT homeostasis and for 3,4-methylenedioxymethamphetamine-induced hyperactivity.