The presence of specific benzodiazepine binding sites in the hearts of dogs and human beings was demonstrated in vivo by a noninvasive method, positron emission tomography (PET). An antagonist of the peripheral-type benzodiazepine binding site, PK 11195, was labeled with carbon-11, a short-lived positron emitter. When injected at high specific activity,11C-PK l1195 was concentrated in the myocardium. As increasing amounts of unlabeled PK 11195 were added to the radioactive ligand, the myocardial ligand concentration was proportional to myocardial regional perfusion up to quantities of 40 nmol/kg body weight. Above 40 nmol/kg the ligand concentration reached a maximum value (6000 pmol/cm3), which could be considered as the total number of binding sites per unit heart volume. The specificity of11C-PK 11195 binding to canine heart was demonstrated from a study on the inhibition of binding for radioligand by an excess of several agonists or antagonists of benzodiazepine receptor. The distribution and specificity of11C-PK 11195 was similar in dogs and in human beings. PET thus opens the way to the investigation of the peripheral-type benzodiazepine receptor in a clinical situation, since it has recently been shown that this receptor could be coupled to the calcium channel in the heart.