a theory advanced by Walter B. Cannon, according to which animal and human organisms in situations requiring that they either fight or flee are provided with a check-and-drive mechanism that puts them in readiness to respond with undivided energy output; the mechanism is characterized by increased sympathetic nervous system activity, including increased catecholamine production with associated increases in blood pressure, heart and respiratory rates, and skeletal muscle blood flow. Thus an internal reaction makes possible external behavior in response to danger.relaxation response. Cf.: alarm reaction SYN: Cannon theory, emergency theory.