Henry, J. P. (Univ. of Southern California, Los Angeles, Calif. 90007) and J. C. Cassel. Psychosocial factors in essential hypertension: Recent epidemiologic and animal experimental evidence. Amer. J. Epid., 1969, 90: 171–200.—Recent developments in the epidemiology of essential hypertension permit more emphasis to be placed on the etiologic role of psychosocial stimulation and early experience. Evidence is presented suggesting that obesity and dietary factors such as salt or fat intake may not be as significant in explaining variations in blood pressure levels in different populations as is the organism's perception of events in the social environment. Animal and human studies are cited which indicate that repeated arousal of the defense alarm response may be one important mechanism involved. In man such arousal follows when previously socially-sanctioned patterns of behavior, especially those to which the organism has become adapted during critical early learning periods, can no longer be used to express normal behavioral urges. Difficulties of adaptation, as where there is status ambiguity, may result in years of repeated arousals of vascular, autonomic, and hormonal function due to the organism's perception of certain events as threatening. These, in turn, can lead to progressive and eventually irreversible disturbances such as renal hypertension, heart failure, or cerebrovascular disease.