In this study, the (+)-enantiomer of radioiodinated 2-[4-(4-iodophenyl)piperidino]cyclohexanol [(+)-[(125)I]-p-iodovesamicol] [(+)-[(125)I]pIV], which is reported to bind with high affinity to sigma-1 receptors in vitro, was tested for its usefulness in imaging sigma-1 receptors in the central nervous system (CNS) in vivo. In biodistribution studies, significant amounts (approximately 3% of the injected dose) of (+)-[(125)I]pIV accumulated in rat brain, and its retention was prolonged. In blocking studies, the accumulation of (+)-[(125)I]pIV in the rat brain was significantly reduced by the coadministration of sigma-ligands such as pentazocine (5.0 micromol), haloperidol (0.5 micromol) or SA4503 (0.5 micromol). The blocking effect of pentazocine (selective sigma-1 ligand) was similar to the blocking effects of SA4503 and haloperidol [nonselective sigma (sigma-1 and sigma-2) ligands]. Ex vivo autoradiography of the rat brain at 45 min following intravenous injection of (+)-[(125)I]pIV showed high localization in brain areas rich in sigma-1 receptors. Thus, the distribution of (+)-[(125)I]pIV was thought to bind to sigma-1 receptors in the CNS in vivo. These results indicate that radioiodinated (+)-pIV may have the potential to image sigma-1 receptors in vivo.