Studies in progress are directed toward uncovering the endocrine and neurocytological bases for the exacerbation of the hypertension, with cardiac, blood vessel, and renal morbidity, which is induced by psychosocial stress in normal and isolation-raised mice (Atherosclerosis, 14:203, 1971). Studies are being conducted to determine if the phenomena observed in mice are also detectable in rats. Normal and isolation-raised rats subjected to prolonged social interactions in a specially designed population cage are being monitored for the development of hypertension and for the occurrence of pathological changes in the heart, blood vessels, and kidneys. Various parameters of adrenal cortical function are also being assessed (blood corticosterone, adrenal cholesterol and ascorbic acid, thymic weights and W.B.C. counts). The effects of a rigorous operant conditioning program are also being analyzed. A newly developed and critical part of our program which is receiving a highest priority is the initiation of parallel studies in squirrel monkeys, Sciamiri sciureus. We are attempting to determine if a sustained hypertension, with accompanying changes in the cardiovascular system and kidney, as well as in adrenal cortical activity, may also be induced in these animals by threatening social conditions and to find out if isolation rearing has any bearing upon the susceptibility to the effects of social hyperreactions.