Summary1. Uninephrectomized, salt-treated rats bearing regenerating adrenals and grouped 3 to a cage developed significantly higher blood pressure than corresponding rats caged singly. Absence of thymic involution in the grouped, adrenal-enucleated rats lends further support to the thesis that the development of hypertensive disease under these conditions is through the secretion of a steroid other than corticosterone. 2. The enhancement of hypertension by crowding occurred in the face of lower dietary and saline consumption and smaller weight gain than was observed in controls housed singly. Similar, but much less consistent changes occurred in intact grouped rats drinking saline or tap water. 3. These observations support the hypothesis that increased population density augments blood pressure levels attained in rats bearing regenerating adrenal glands.