He studied stress effects by subjecting lab mice to various physical, antigenic, and environmental stressors, including excessive exercise, starvation, and extreme temperatures. He determined that regardless of the type of stress, the mice exhibited similar physical effects, including thymus gland deterioration and the development of ulcers.[2] Selye then developed his theory of general adaptive syndrome (GAS) in 1936, known today as "stress response". He concluded that humans exposed to prolonged stress could also experience hormonal system breakdown and subsequently develop conditions such as heart disease and elevated blood pressure.[3] Selye considered these conditions to be "diseases of adaptation", or the effects of chronic stress caused by heightened hormonal and chemical levels.[2] His research on acute and chronic stress responses introduced stress to the medical field.[2]