A specific radioimmunoassay for angiotensin II has shown that its normal concentration in arterial blood is 2.4±1.2 (S.D.) mμg./100 ml.; the venous level is consistently below this value, being usually 50–75% of it. Definite rises in blood angiotensin II levels were found in some patients with hypertension, both essential and secondary to renal disease. Extremely low levels were observed in two anephric women, and in one patient with Conn's syndrome. This radioimmunoassay offers a valuable alternative to renin bioassay in evaluation of the role of the renal pressor system in clinical disorders associated with hypertension and aldosteronism.