Studies in rodents have shown that psychostimulant drugs such as cocaine and amphetamine cause endorphin release in the brain reward system. There is also evidence for the involvement of the opioid system in human psychostimulant dependence. The acute effects of an i.v. psychostimulant drug on the brain opioid system, however, have not yet been investigated in humans. We hypothesized that an i.v. dose of amphetamine as compared to placebo would cause an opioid release in the human brain reward system, measurable as a reduction of the binding potential of the μ-opioid receptor radioligand [11C]carfentanil. Ten healthy young men were examined using positron emission tomography (PET) and [11C]carfentanil in three sessions: at baseline; after placebo; after an i.v. amphetamine dose of 0.3 mg/kg bodyweight. The order of amphetamine and placebo was double-blinded and randomized. PET examinations were performed with a Siemens high resolution research tomograph. Data were analysed with the simplified reference tissue model, applying manually drawn regions of interest for every subject. Using repeated measures analysis of variance, we found no significant differences in [11C]carfentanil binding potential between amphetamine and placebo conditions in any of the investigated brain regions. In contrast to data from rodent studies and a recent study of oral amphetamine administration in humans, an i.v. dose of amphetamine does not cause any acute opioid release in healthy human subjects. The postulated role of the opioid system in mediating the effects of amphetamine needs to be further investigated in animal models of the disease as well as in patient populations.