Abstract: As with most mammals, domestic livestock will experience varying degrees of both psychological and physiological stress at some time during their life. The objective quantification of these stressful states and application of appropriate measures to limit excessive exposures to stressors is imperative. Proper management of an animal`s exposure to stressors will maximize animal well-being and can have beneficial effects on animal production. Although scientists have recognized the deleterious effects of distress for more than 70 years, debates and questions on physiologically assessing its presence in humans and other animals continues to challenge researchers today. Because stress can be defined simply as any physiological change from homeostasis, traditional physiological measurements have relied on quantifying these alterations to homeostasis, such as deviations in heart rate, respiration rate, body temperature, and hormone concentrations. These measurements are still highly relied upon today. It is also well recognized that when these common alterations in baseline homeostatic mechanisms are dramatically altered, organism life strategies such as growth, disease resistance, and reproduction can be affected. Therefore, a great deal of research has concentrated on quantifying physiological alterations in these systems, such as changes in growth and reproductive hormones, changes in populations of lymphocytes, and (or) outward signs of failure of these systems, such as low growth rates, infertility, and an increased number of diseased animals. An area of importance that has been relatively inaccessible are those changes that occur in the central nervous system. Because stress is commonly composed of both a physiological and psychological component, how the animal perceives the stress is critical to assessing its well-being. Physiological measures used thus far to assess the mental response to stress include neuronal activity and measurement of neurotransmitters. Scientists have done a good job of measuring all of these physiological alterations, unfortunately the underlying challenge that continues to confront scientists is how to define the degree of physiological change that translates into distress for the animal.