There is good evidence for the interaction of neuroendocrine and immune systems. Endotoxin (LPS)-induced mediators (e.g., cytokines, prostaglandins etc) set on endocrine organs (e.g., the hypothalamo-pituitry-adrenal axis; thyroid glands etc). Endotoxin-treated, intestinal ischemic, or irradiated rats show decreased T 4 levels of blood. These animals cannot respond to TSH because the TSH-receptors of follicular membranes are disturbed by LPS in the thyroid glands. Radiodetoxified endotoxin is an effective immunstimulator and does not disturb the follicular membrane of thyroid gland. Thus, the T 4 production remains normal. The bile acids--as the end-product of cholesterol metabolism--play an important role in the physiological defense of macroorganisms against endotoxin and other lipid-like agents (Physico-chemical defense) and in the regulation of endocrine system, including the reproduction.