The relative importance of salt intake and psychosocial stimulation in the development of high blood pressure has been studied in colonies of CBA/USC mice. Approximately 50 males were observed for 3–4 months in five population cages which successfully induced chronic psychosocial interaction, resulting in chronic hypertension. Under these conditions, progressive arteriosclerosis develops together with myocardial hypertrophy, increased catecholamine synthesis and increased angiotensin sensitivity. Previous work indicates that this condition shows the characteristics of renin dependent human hypertension. A special grain based diet was used which included 0.014% sodium. This resulted in the ingestion of the equivalent of 40 mmol/l sodium or 3.0 g NaCl in a 70-kg man. This, and an even more stringent synthetic diet containing < 0.01 % NaCl, i.e. < 2 g NaCl per day in man, were contrasted with the standard chow which contains 0.4% sodium.Over 4 months of social interaction the psychosocial stimulation proved to be the critical factor and, despite the low-salt intake, blood pressure rose to the same levels as those of control groups on a normal diet containing 1 % salt. Hypertension occurs in the absence of kidney failure as assessed by blood urea. Plasma renin levels on the low-salt grain based diet were double those on standard chow, showing that the diet was sufficiently low in salt to activate the renin-angiotensin system.