Angiotensin-(1-8)octapeptide (angiotensin II) is the active principle of the renin-angiotensin system. Crossreaction of angiotensin II-antisera with inactive precursors and metabolic fragments prevented the specific quantitation of this hormone in biological fluids. Peptide-extraction on bonded-phase silica followed by peptide-separation using isocratic reverse-phase high performance liquid chromatography and subsequent radioimmunoassay rendered possible the octapeptide-specific measurement of angiotensin II in 2 ml plasma with a detection limit of 0.4 fmol/ml. The coefficient of variation for intra-assay precision was 0.06 and for inter-assay precision 0.13. 125I-angiotensin II was recovered from plasma by solid-phase extraction to 99 +/- 2% (mean +/- S.D.). The overall recovery of 5, 10 and 20 fmol unlabeled angiotensin II added to plasma was 80 +/- 10%. Plasma concentrations in supine normal humans averaged 4.1 +/- 1.6 fmol/ml and were suppressed below the detection limit by angiotensin I converting enzyme inhibition.