Previous work suggests that iomazenil (formerly known as Ro 16-0154) is a useful ligand for static imaging of the benzodiazepine (BZ) receptor with single photon emission computed tomography (SPECT). The present study evaluated the feasibility of dynamic SPECT imaging of cerebral radioactivity following intravenous [123I]iomazenil injection in healthy human subjects, in preparation for future receptor quantitation studies. Maximal brain uptake was reached approximately 25-30 minutes after i.v. administration of the radioligand and represented approximately 12% of the injected dose. The regional distribution of radioactive densities was consistent with the known distribution of BZ receptors in human brain, with highest uptake localized over the occipital area. Washout of cortical radioactivity was regionally variable but relatively slow, with a half-life of approximately 4 hours after the time of peak radioactivity. In summary, [123I]iomazenil is a promising SPECT radioligand for the BZ receptor, with high brain uptake, relatively slow washout of radioactivity, and appropriate regional distribution.