Using in vivo microdialysis techniques, the effects of RTI-55 and/or cocaine on extracellular dopamine (DA) concentrations were measured in the nucleus accumbens (NACC) of freely moving rats. In control animals, cocaine (20 mg/kg) increased NACC DA approximately 458% 60 minutes following administration, returning to baseline values within 200 minutes. Similarly, RTI-55 administration (0.25 mg/kg) increased NACC DA levels approximately 347%. When combined, however, cocaine further increased NACC DA to 705% of baseline values when given 4 hours following RTI-55. This increase was significantly larger than cocaine alone (P < 0.05). In addition, chronic RTI-55 administration (5 days) further potentiated cocaine's ability to increase NACC DA (783%) but this did not reach statistical significance (P > 0.1) compared to acute RTI55/cocaine animals. These findings indicate that RTI-55, a drug that binds directly to the dopamine transporter (DAT) with higher affinity than cocaine, does not appear to be effective in attenuating cocaine's effects on NACC dopamine levels. In fact, acute RTI-55 potentiates cocaine's effects on NACC DA.