Calcitonin has been shown to affect calcium handling within cells thus impairing insulin secretion and glucose tolerance in healthy subjects. In the present study we investigate the effects of calcitonin on basal and nutrients-induced plasma glucose and insulin levels variations in healthy subjects (n = 10) and in patients affected by islet cell tumor (n = 6). In healthy subjects calcitonin markedly decreased basal and nutrients-induced plasma insulin levels while in patients with islet cell tumor this calcitonin-mediated effect was lost. So we conclude that the lack of calcitonin effect upon insulin secretion in patients with insulinoma is probably due to the autonomous insulin secretion characterizing islet cell tumor.