In connection with clinical 131I-MIBG studies of patients with suspected pheochromocytoma and adrenomedullary hyperplasia quantitative biokinetic data have been collected in order to improve the present estimations of absorbed dose to various organs and tissues. Whole-body profiles as a function of time were measured with a whole-body counter. The retention in the total body and in the thyroid gland could be derived from the measured whole-body profiles by summing up the corresponding values. The retention of 131I-MIBG could not be exactly measured for further organs from the whole-body profiles in man. For this reason animal studies were performed with mice. The biokinetic animal data were transferred to man in form of the cumulative activity for the various organs. The mean absorbed dose for selected organs per injected activity unit was calculated using the concept of absorbed fractions (MIRD method) taking into account the radioactivity within the remaining body. Except for both the adrenal medulla and the thyroid gland the absorbed doses for all the other selected organs are in a range from 0.108 mGy MBq-1 for the testes to 0.176 mGy MBq-1 for the lungs. The absorbed dose to the thyroid gland amounts to the considerable value of 5.69 mGy MBq-1 although the thyroid gland was blocked. The greatest absorbed dose was estimated for the normal adrenal medulla with 18.67 mGy MBq-1.