A wide variety of different stressful stimuli will stimulate corticotropin release from the pituitary gland. Two hypothalamic hormones, corticotropin-releasing hormone and arginine vasopressin, have a synergistic action in stimulating corticotropin release. However, the pattern of corticotropin-releasing hormone, arginine vasopressin, and corticotropin secretion varies with the type and chronicity of the stress. Inhibition of corticotropin secretion by cortisol and opioid peptides may also affect the molar ratios of hypothalamic-pituitary-adrenal (HPA) axis hormones. Catecholamines acting through central α1 adrenoreceptors are thought to be very important in stimulating the HPA axis. However, peripheral catecholamine concentrations do not necessarily correlate with HPA axis hormones. The relationship between the immune system and the hormones of the HPA axis is a very rapidly advancing area of knowledge in the endocrinology of stress.