"Hans Selye (1907–1982) started the modern era of research into something called stress. In 1950, Selye addressed the American Psychological Association convention and described a theory of stress-induced responses that became the standard model of stress. Selye's discovery of the stress response was an accident. He was researching the effects of hormone injections in rats and initially thought the hormones caused harm because many rats became sick after receiving the injections. However, when he used a control group injected only with a neutral solution, those rats became sick too. The rats responded more profoundly to the trauma of being injected than to the hormones. Being handled and injected led to high levels of sympathetic nervous system arousal and eventually to health problems such as ulcers. Selye coined the term \"stressor\" to label a stimulus that had this effect. The immediate response to stress is the release of adrenaline into the blood plasma. Mild stressors such as opening a cage door or handling a rat produce an eightfold increase in plasma epinephrine, or adrenaline, concentrations."