The feasibility of imaging cerebral opioid receptors by single photon emission computed tomography (SPECT) has been established in baboon using a novel analog of diprenorphine (DPN) radiolabeled with iodine-123. The radioligand, [ 123 I]-O-IA-DPN (C6-O-[ 123 I]iodoallyl-DPN), was prepared in good yield (80%) with high radiochemical purity (Ͼ97%) and high specific radioactivity (Ͼ2,400 mCi/µmol). In ex vivo autoradiographic studies, with and without naltrexone blockade, [ 123 I]-O-IA-DPN specifically labeled opioid receptors throughout the mouse brain. Nonmetabolized radioligand accounted for Ͼ90% of the signal observed in extracts of whole mouse brain. SPECT imaging trials showed that [ 123 I]-O-IA-DPN selectively localized in regions of baboon brain known to have high densities of opioid receptors, such as striatum, thalamus, and temporal cortex. A much lower level of radioligand uptake and retention was noted for cerebellum, a region with few opioid binding sites. Pretreatment with naltrexone (6.5 µmol/kg) blocked [ 123 I]-O-IA-DPN binding in all brain regions. Using naltrexone blockade to define the nonspecific component for a given region of interest, total to nonspecific binding ratios increased linearly (r Ն 0.98) over the SPECT study with maximal values for striatum (9.8), thalamus (7.1), and temporal cortex (6.9) reached at the last time point investigated (3.5 h). Specific binding for these regions, assessed as the difference between regional SPECT activity for the control and blocked states, proved irreversible over the observation period. By the end of the time course, specific [ 123 I]-O-IA-DPN binding was Ͼ85% of total radioactivity in regions rich in opioid receptors and 62% of total radioactivity in cerebellum. The aggregate data are consistent with visualization of multiple opioid receptor types. Thus, [ 123 I]-O-IA-DPN should prove useful for SPECT studies within the constraints imposed by a lack of innate selectivity for a single type of brain opioid receptor.