Renin activity in blood plasma was studied by the radioimmune method in healthy persons and in hypertensive disease patients of various ages. It was found that this index diminishes with age in physiological ageing. The increase in the sensitivity of the ageing body to humoral agents, angiotensin among others, evidently compensates for the qualitative deficiency of the hormone. The results obtained provide evidence that the renal pressor mechanism is involved in the pathogenesis of late hypertensive disease at an earlier stage, which is probably due to the age preconditions, particularly the diminished intensity of intrarenal circulation, relative sympathicotonia at elderly and old age, and features of the electrolyte metabolism. The early activation of the renin--angiotensin system in the late stages of hypertensive disease should determine specified pathogenetic treatment of this disease in old age.