Stress involves real or perceived changes within an organism in the environment that activate an organism's attempts to cope by means of evolutionarily ancient neural and endocrine mechanisms. Responses to acute stressors involve catecholamines released in varying proportion at different sites in the sympathetic and central nervous systems. These responses may interact with and be complemented by intrinsic rythms and responses to chronic or intermittent stressors involving the hypothalamic-pituitary-adrenal axis. Varying patterns of responses to stressors are also affected by an animal's assessment of their prospects for successful coping. Subsequent central and systemic consequences of the stress response include apparent changes in affect, motivation, and cognition that can result in an altered relationship to environmental and social stimuli. This review will summarize recent developments in our understanding of the causes and consequences of stress. Special problems that need to be explored involve the manner in which ensembles of adaptive responses are assembled, how autonomic and neurohormonal reflexes of the stress response come under the influence of environmental stimuli, and how some specific aspects of the stress response may be integrated into the life history of a species.