Cortisol is the principal active glucocorticoid produced by the human adrenal gland, which is the key regulator of whole-body homeostasis. Cortisol is essential for provision of nutrients to tissues via carbohydrate, protein, and lipid metabolism. It also affects all of the major systems of the body, including the cardiovascular, musculoskeletal, nervous, and immune systems, and play critical roles in fetal development. Secretion of cortisol is controlled by hypothalamus, pituitary and the adrenal gland. CRH released from the hypothalamus acts on pituitary gland to release ACTH, which acts on the adrenal gland to increase the synthesis and release of cortisol. This book focuses on the changes in plasma cortisol observed in adult male rabbits treated with monosodium glutamate. Monosodium glutamate, is the salt which is frequently used in daily life. This book demonstrates that how monosodium glutamate crosses the blood brain barrier; acts on the HPA axis; and consequently increases the plasma cortisol level.