Evidence from animal studies suggest that centrally acting opiates increase synaptic dopamine (DA) concentration. However, the interaction between mu-opioid receptors and the DA system is unclear. We report here an effect of fentanyl on striatal [123I]beta-CIT binding to the DA transporter in a patient and in rats. A female patient underwent [123I]beta-CIT single-photon emission tomography (SPET) study after intrathecal injection of fentanyl for her back pain. After a 2-week drug-free period, the SPET study was repeated. In the experimental study, male Wistar rats were treated with fentanyl either acutely (50 micrograms/kg, i.p.) before imaging study or subacutely for 4 days (10 micrograms/kg, twice a day, i.p.). Brain planar imaging was performed at 3.5 hours after an intravenous injection of [123I]beta-CIT with gamma camera with a pinhole collimator. In a female patient, [123I]beta-CIT binding in the basal ganglia was decreased by 37% during fentanyl as compared to the binding after 2-week drug-free period. Similarly in rats, acute fentanyl treatment decreased [123I]beta-CIT binding to the striatum by 30% as compared to that of with the control rats. After subacute administration of fentanyl, no significant difference was observed compared to the control group. According to the present data, fentanyl decreases [123I]beta-CIT binding in the basal ganglia both in human and rats, suggesting that opiates possibly directly affect DA reuptake.