In hypertension associated with renal artery stenosis, the evolution of the raised blood pressure can conveniently be considered in three phases. In the first phase, blood pressure is raised by the direct pressor action of elevated peripheral plasma angiotensin II. In the second phase, circulating angiotensin II may be more modestly raised, but probably is still important in pathogenesis. Occasionally in phase II there is rapidly advancing elevation of renin, angiotensin II and aldosterone and severe hypertension, with sodium and potassium depletion. In the much later third phase, angiotensin II is not elevated, and the renin system may no longer be concerned in the hypertension. In phases I and II, but not in phase III, relief of the stenosis, removal of the affected kidney, or lowering of angiotensin II with converting enzyme inhibitors, can correct the hypertension. In the affected kidney with renal artery stenosis, the intrarenal content of renin is raised and its distribution altered; these changes represent compensatory local actions. The affected kidney secretes both active and inactive renin, while there is suppression of renin secretion by the contralateral kidney which becomes a net extractor of angiotensin II.