The tissue distributions of three radioiodinated neuron-blocking agents have been determined in dogs. Iodine-125-labeled meta- and para-iodobenzylguanidines show a striking affinity for, and retention in, the adrenal medulla. Peak concentrations of the two isomers exceed those of previously reported adrenophilic compounds. High myocardial concentrations were also observed at early time intervals. Images of the dog's adrenal medullae have been obtained with para [131I]iodobenzylguanidine.