Much of the action of the body in maintaining cardiovascualr, gastrointestinal, and thermal homeostasis occurs through the autonomic nervous system (NAS). The ANS is also our primary defense against challenges that homeostasis. It provides involuntary (outside of consiciousness) control and organization of both maintenance and stress responses. In the words of Claude Bernard, the nature through it provident to remove these important phenomena from the capariciousncess of an ignorant will. Activation of the sympathetic nervous system elicits what is traditionally called the "fight of flight" response, including redistribution of blood flow from the viscera to skeletal muscle, increased cardiac fucntion, sweating, salivation, and papillary dilation. The parasympathetic system governs activities of the body more closely associated with maintenance of fucntion, such as digestive and genitourinary fucntions. A major goal of anesthetic administration is maintaining optimum homeostasis in the patient, inspite of powerful challenges to a sometimes tenuous physiologic balance. The intelligent administration of anesthetic care to patients requires a knowledge of ANS pharmacology in order to achieve desirable interactions anesthetic with the involuntary control system and to avoid responses or interactions with deleterious effects. In addition, disease states may impair ANS fucntion to a significant extent and may thereby alter the expected responses to surgery and anesthesia. Last, possible negative effects of the human stress response have long been appreciated, and considerable effort has been expended in examining the possibility that modification or ablation of the stress response may improve perinoperative outcome.