We investigated the transport of cationic neurotoxins and neurotransmitters by the potential-sensitive organic transporter OCT3 and its steroid sensitivity using heterologous expression systems and also analyzed the expression of OCT3 in the brain. When expressed in mammalian cells, OCT3 mediates the uptake of the neurotoxin 1-methyl-4-phenylpyridinium (MPP+) and the neurotransmitter dopamine. Competition experiments show that several cationic neuroactive agents including amphetamines interact with OCT3. When expressed in Xenopus laevis oocytes, OCT3-mediated MPP+ uptake is associated with inward currents under voltage-clamp conditions. The MPP+-induced currents are saturable with respect to MPP+ concentration, and half-maximal saturation (K0.5) occurs at about 25 microM MPP+ with membrane potential clamped at -50 mV. The K0.5 for MPP+ is markedly influenced by membrane potential. OCT3 is inhibited by several steroids, and beta-estradiol is the most potent inhibitor (Ki approximately 1 microM). The pattern of steroid sensitivity of OCT3 is different from that of OCT1 and OCT2 but correlates significantly with that of the extraneuronal monoamine transporter (uptake2). The transport characteristics and steroid sensitivity provide strong evidence for the molecular identity of OCT3 as uptake2. OCT3 is expressed in the brain as evidenced from Northern blot analysis, reverse transcription-polymerase chain reaction, and in situ hybridization using OCT3-specific probes. The molecular identity of the transcript hybridizing to the probe has been established by sequencing the reverse transcription-polymerase chain reaction product and also by the isolation of the OCT3 cDNA from a brain cDNA library. Regional distribution studies with in situ hybridization show that OCT3 is expressed widely in different brain regions, especially in the hippocampus, cerebellum, and cerebral cortex. OCT3 is likely to play a significant role in the disposition of cationic neurotoxins and neurotransmitters in the brain.