A sensitive and specific radioimmunoassay for the determination of plasma angiotensin II was developed by using the antisera against synthetic angiotensin II in combination with labeled angiotensin II. This assay employs an acetone extraction procedure and detects as little as 0.8 pg per tube of angiotensin II. The mean (+/- S.E.) plasma angiotensin II concentration in 19 normal subjects was 14.4 +/- 1.8 pg/ml in a state of overnight fasting and recumbency. In 13 normal subjects, in whom 40 mg of furosemide was injected intravenously, plasma angiotensin II concentration before and after 30 and 120 minutes in an upright position was 14.6 +/- 2.2, 56.6 +/- 5.7 and 74.3 +/- 9.0 pg/ml, respectively. In 6 normal subjects, an infusion of isotonic saline, angiotensin II concentration reduced from 14.1 +/- 3.7 to 9.2 +/- 1.7 pg/ml. Thus, it was ascertained that the simplified radioimmunoassay method reported here using an acetone-petroleum ether extraction method was specific and highly sensitive.