The pheochromocytoma-scanning agent 131I-meta-iodobenzylguanadine (131I-MIBG) was concentrated in the platelets of ten subjects undergoing scans for suspected pheochromocytomas. In vitro studies also demonstrated that 131I-MIBG was concentrated in platelets. In contrast to serotonin, which had maximum in vitro platelet uptake of one hour, the peak in vitro platelet uptake of 131I-MIBG did not occur until six hours. In vitro platelet 131I-MIBG uptake was decreased by 79% to 98% following the neuronal-uptake-blocking agent imipramine. These studies suggest that pheochromocytomas take up 131I-MIBG by a neuronal-pump mechanism. Further studies to determine if 131I-MIBG is taken up by other tumors with a neuronal-pump mechanism, such as carcinoid tumors, will be valuable.