Because serotonergic function has been implicated in the pathophysiology of a number of diseases of the nervous system, efforts to image this system in vivo have received considerable recent attention. Promising preliminary results with the tracer 5-iodo-6-nitroquipazine (INQUIP) have prompted us to perform further studies designed to validate the use of the tracer as an in vivo ligand for the serotonin transporter. METHODS We studied six adult macaca mulatta in eight experiments which involved SPECT imaging at 17 to 24 hr post-tracer injection, including three experiments with coinjection of the 123I-and 125I-radiolabeled tracer for direct comparison of autoradiography and SPECT, and three experiments in which animals were lesioned with the serotonergic neurotoxin (+/-)3,4-methyl-enedioxymethamphetamine (MDMA). In addition, we evaluated the metabolism of the tracer in the brain and periphery. RESULTS SPECT images obtained at 17 and 24 hr reflected the known pattern of distribution of serotonin transporters and also showed close correspondence to the autoradiograms. Ratios of binding in the brain-stem to binding in the cerebellum were close to 3 at 17 hr. autoradiograms from an MDMA-treated animal showed up to 95% reductions of binding, while the SPECT data showed smaller reductions. Virtually all of the tracer in the brain stem was in the form of unmetabolized parent compound, but plasma showed rapid peripheral metabolism of the tracer. CONCLUSIONS These results demonstrate that INQUIP SPECT images are sensitive measures of in vivo binding to the serotonin transporter, and support the further development of the tracer as a method for the in vivo study of serotonergic neurons in humans.