Microdialysis was used to investigate the effects of heroin administration on extracellular opioid peptide levels in the globus pallidus/ventral pallidum of freely moving rats. Two injections of heroin (0.6 mg/ kg i.p.) were given 3 h apart. The first injection had no significant effect on opioid peptide levels, but the second injection produced a transient yet significant increase (+268%) in opioid peptide-like immunoreactivity in pallidal dialysates, peaking 1 h after injection. This effect was blocked by administration of naloxone (3 mg/kg i.p.) prior to the second injection. The implications of these data are discussed with regards to the role of the endogenous opioid peptide system in opiate reward.