To assess a possible heritability of a disturbed calcium metabolism in relation to blood pressure regulation, 28 young normotensive offspring of either hypertensive or normotensive parents were studied while administered a denned diet with daily sodium chloride of 6 and 20 g/day for 7 days each. Before the high salt diet was begun, the cytosolic calcium concentration ([Ca2+]1) in platelets was elevated in offspring of hypertensive parents, whereas serum electrolytes, plasma renin activity, plasma catecholamines, and 24-hour urinary excretion of sodium and calcium showed no difference between the two groups. On exposure to a high salt diet, the mean blood pressure increased (from 80±2 to 85±2 mm Hg,p<0.05) in offspring of hypertensive parents. These changes in mean blood pressure were positively correlated with the basal platelet [Ca2+]1(r=0.61,p<0.01), whereas [Ca2+]1did not demonstrate any significant changes. When the subjects were administered the high salt diet, plasma ionized calcium decreased (from 2J7 to 2.21 meq/1,p<0.05) and 1, 25 -dihydroxy vitamin D3 increased (from 32.7 to 40.8 pg/ml,p<0.05) with a transient relative hypercalciuria in offspring of hypertensive parents. This increase of 1, 25 -dihydroxyvitamin D3 was significantly correlated with the changes in mean blood pressure (r=0.62,p<0.01). Disturbed intraplatelet and systemic calcium metabolism may be of predictive value in the development of hypertension.