To measure the octapeptide angiotensin II (Ang II) in plasma, we developed a sensitive, specific assay that interfaces solid-phase extraction, HPLC, and RIA. A reversed-phase HPLC system involving isocratic elution at 38 degrees C with a volatile mobile phase of acetonitrile and the ion-pairing reagent heptafluorobutyric acid produced baseline separation of angiotensin peptides. Ang II was collected as a single fraction, concentrated by evaporation to dryness, and measured by RIA after resuspension in RIA buffer. Even including column washing between sample injections to prevent carryover of plasma constituents, two plasma extracts could be processed per hour by HPLC. Assay validation experiments demonstrated < 2% cross-reactivity with Ang II-related peptides; a 75% recovery from plasma at physiological concentrations of Ang II; intra- and interassay precision (CVs) of 6.2% and 10.3%, respectively; and a lower limit of quantification of 1.3 ng/L. Two clinical protocols designed to measure plasma Ang II concentration under basal and stimulated conditions confirmed the utility of the assay.