Tests for demonstrating the responsiveness of plasma renin activity (PRA) have been simplified and their validity for the diagnosis of primary aldosteronism was examined. The PRA after various manuevers which shrink plasma volume differentiates normotensive control groups from patients with primary aldosteronism. A single specimen after four hours' upright posture while on a normal diet is of good discriminatory value in assessing renin responsiveness. However, suppressed PRA is not proof of the diagnosis of primary aldosteronism. One fourth of 47 patients considered to have essential hypertension had suppressed PRA. Using other criteria for the diagnosis of primary aldosteronism, these nonresponsive hypertensive patients do not have primary aldosteronism. The mechanism for their suppressed PRA is unknown. The PRA measurements are of value in the exclusion of primary aldosteronism, but other evidences of aldosterone excess must be found to make this diagnosis.