Calcitonin (CT), a 32-aminoacid peptide, is secreted by the parafollicular 'C' cells of the thyroid which derive from the ultimobranchial body and, ultimately, from the neural crest. In man, the main role of this hormone is to protect the skeleton during periods of physiological stress, such as growth, pregnancy and lactation by reducing calcium loss [1]. Nevertheless, some recent data, such as the finding of immunoreactive CT-like material (iCT) in the central nervous system [2] and in the cerebrospinal fluid and the demonstration of its analgesic action in rabbits [4] and in man [5] indicate a relationship between CT and the neuroendocrine system. The purpose of the present study is to evaluate the levels of CT in a group of addicts to heroin both before and during detoxication treatment with methadone or a non-opioid drug like clonidine.