Human organic cation transporter 2 (hOCT2) is involved in transport of many endogenous and exogenous organic cations mainly in kidney and brain cells. Since the quaternary structure of transmembrane proteins plays an essential role for their cellular trafficking and function, we investigated whether hOCT2 forms oligomeric complexes and if so which part of the transporter is involved in the oligomerization. A mating-based split-ubiquitin-yeast-two-hybrid system (mBSUS), fluorescence resonance energy transfer, Western blot analysis, crosslinking experiments, immunofluorescence, and uptake measurements of the fluorescent organic cation 4-4-(dimethylamino)styryl)-N-methylpyridinium were applied to HEK293 cells transfected with hOCT2 and partly also in freshly isolated human proximal tubules. The role of cysteines for oligomerization and trafficking to the plasma membranes was investigated in cysteine mutants of hOCT2. hOCT2 formed oligomers both in the HEK293 expression system and in native human kidneys. The cysteines of the large extracellular loop are important to enable correct folding, oligomeric assembly, and plasma membrane insertion of hOCT2. Mutation of the first and the last cysteines of the loop at position 51 and 143 abolished oligomer formation. Thus, the cysteines of the extracellular loop are important for a correct trafficking to the plasma membrane and for oligomerization. Supported by the Deutsche Forschungsgemeinschaft (DFG CI 107/4-1 to 2).