The serotonin transporter (SERT) plays a critical role in the maintenance of normal neurotransmission by serotonin [5-hydroxytryptamine (5-HT)]. Recent evidence suggests that SERT and other neurotransmitter transporters are tightly regulated. Activation of protein kinase C results in a decrease in SERT-mediated 5-HT uptake, which is due to an internalization of the transporter. However, to date little is known about the mechanism and proteins involved in the down-regulation of the transporter. One candidate SERT-regulatory protein is the SNARE (soluble N-ethylmaleimide-sensitive factor-attachment protein receptor) protein, syntaxin 1A (Syn1A), which has recently been implicated in the regulation of ion channels as well as the SERT-related gamma-aminobutyric acid- and glycine-transporters. Using 5-HT uptake assays, confocal microscopy and glutathione S-transferase (GST) pull-down assays we showed that Syn1A also interacts with SERT and alters the subcellular localization of the transporter, resulting in a reduction of 5-HT transport. In addition, we have used the yeast two-hybrid system to search for novel regulatory proteins that interact with the cytoplasmic N-terminal domain of SERT. By screening rat brain cDNA library we have identified six potential SERT-binding proteins. Here we also present progress towards the elucidation of the biological relevance of these proteins and their potential role for the regulation of the serotonin transporter.