Uptake of (+/-)-[3H]-noradrenaline, [14C]-bretylium and [125I]-o-iodobenzyltrimethylammonium iodide (RIBA) by rat heart was studied by the Langendorff technique. All three compounds showed significant uptake. 2 Corticosterone and 17-beta-oestradiol inhibited the uptake of all three compounds by rat heart, a finding consistent with extraneuronal uptake (uptake2). 3 [131I]-RIBA was injected intravenously into pigs and monkeys (M. speciosus). Myocardial samples taken from pigs killed 1 and 2 h after injection showed significant uptake. No significant uptake was found in myocardial samples of monkeys killed 10 min, 2 h and 24 h, respectively, after injection. 4 Four normal human volunteers received [125I]-RIBA intravenously and the image of the precordial area was followed by means of scintillation camera for the first 4 h after injection. In two of the subjects, the scintigrams were repeated at 22 and 23 h after injection, respectively. No evidence of myocardial uptake was observed. 5 These results suggest the possibility that man and at least one other primate species may differ from lower species with regard to uptake.