Organic cation transporters (OCT) are expressed in the plasma membrane of epithelial cells from a wide range of tissues, where they function in the elimination of endogenous amines, cationic drugs and other xenobiotics. In humans, OCT1 is primarily expressed in the liver while OCT2 is expressed in the kidney. OCT3 is expressed in the placenta, skeletal muscle, prostate, aorta and liver. OCT3, also known as extraneuronal monoamine transporter, is widely expressed in different regions of the brain including the hippocampus, cerebellum and cerebral cortex. OCT3 mediates the uptake of several neuroactive agents, including dopamine, and may play an important role in the disposition of neurotransmitters and cationic neurotoxins in the brain. The genes encoding human OCT1–3 map to a conserved cluster at chromosome 6q26-q27.