Meta-123I-iodobenzylguanidine (123I-MIBG), which is an analog of norepinephrine, can be used to evaluate the integrity and function of sympathetic nerve endings in the heart. Myocardial uptake of 123I-MIBG was studied in 30 myocardial infarction patients and compared with the distribution of blood flow assessed with 201Tl. It was found that when a cold defect appeared on the 201Tl scintigram, its localization was identical to the cold defect on the 123I-MIBG scintigram. On the other hand, in three cases, a defect was found on the 123I-MIBG scintigram, corresponding to the electrocardiographic localization of the infarct, whereas the 201Tl scintigram was normal. Most strikingly, the present study shows that drugs (antagonists of the adrenergic receptors, calcium antagonists, amiodarone) decrease or even abolish (as in the case of labetalol) myocardial uptake of 123I-MIBG. Consequently, any interpretation of the 123I-MIBG scintigram must take into account the treatment administered.