The hERG potassium channel is a voltage-gated potassium (K_v) channel, which plays a key role in the repolarisation of the cardiac action potential. Genetic mutations in the hERG channel have been shown to cause long QT syndrome type 2 (LQT2) by several mechanisms; some alter the functional properties of the channel while others impair the trafficking of newly synthesised hERG protein from the ER to the membrane (1). The density of cell surface receptors and channels is regulated not only by the biosynthetic pathways, but also via endocytic mechanisms and these processes play an important role in cell physiology (2). The fate of the hERG channel subsequent to membrane insertion is, however, currently unknown. Here we investigated whether the hERG potassium channel undergoes endocytosis and recycling. hERG channels containing a HA (haemagglutinin A) epitope introduced into an extracellular loop were expressed either transiently or stably in HEK 293 cells. The cell surface channels were first bound with anti-HA antibody at 4°C and their trafficking was followed at 37°C using standard immunocytochemistry and confocal microscopy. Results show that internalisation of hERG channels occurred within 30 min of incubation at 37°C. Internalisation could be prevented by either K⁺ depletion or hypertonicity (induced with 0.45 M sucrose). Both K⁺ depletion and hypertonicity are known to specifically prevent clathrin-mediated endocytosis. Internalisation of Alexafluor488-conjugated transferrin was also prevented under these conditions indicating that the hERG channel undergoes endocytosis via clatharin-coated vesicles. We next examined if the internalised hERG channels were able to recycle back to the plasma membrane. For this we allowed the HA antibody labelled channels to internalise for 2 hrs; cells were then stripped of remaining surface antibody and kept at 37°C to follow recycling. The results showed that the labelled channels returned to the cell surface within 15 min. This recycling could be prevented by pre-treatment of cells with 60 μM primaquine, a known blocker of protein recycling. These data suggest that the internalised hERG channels are able to recycle back to the plasma membrane. To investigate the route of endocytic trafficking, we have co-transfected the hERG channel with GFP-conjugated markers for various endocytic compartments. Co-localisation was observed with markers of early and sorting endosomes and the endocytic recycling compartments. Co-localisation was also observed with late endosomal and lysosomal markers. These data suggest that while some of the internalised channels are able to recycle, others may undergo lysosomal degradation. In conclusion, our data provide evidence for endocytosis and recycling of the hERG channel. The physiological importance of endocytic trafficking of hERG has yet to be elucidated.