When some metabolic changes occur--namely, when blood levels of free fatty acids (F.F.A.), insulin, cholesterol, or triglycerides rise--cellular immunity becomes depressed. These metabolic changes also promote the division of somatic, non-lymphoid cells, which in turn increases the possibility of malignant transformation. The diametrically opposite effects of these metabolic changes on non-lymphoid cells, on the one hand, and on T-dependent lymphocytes and macrophages on the other, increase an individual's susceptibility to cancer. This normally occurs with pregnancy, rapid growth, stress, and ageing. Many chemical carcinogens raise the hypothalamic threshold to feedback control mechanisms, decrease glucose tolerance, and increase blood-insulin--i.e., all the main changes of "cancer-proneness". Changes in biogenic amine level in the hypothalamus during ageing and stress and after an exposure to some carcinogens are responsible for the elevation of hypothalamic threshold to feedback control.