The potential binding of angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe) (AII) to a peptide encoded by its complementary RNA (Lys-Gly-Val-Asp-Val-Tyr-Ala-Val) (IIA) has been studied by monitoring the 1H NMR spectrum of IIA in aqueous phosphate or Tris.HCl buffer (2H2O) as it is titrated with AII. For molar ratios of AII/IIA ranging from 0.2 to 1.8, the NMR spectra are unchanged as compared to the spectra of the isolated peptides. Based on these findings, the Kd for the putative biomolecular complex of the two peptides under these conditions is calculated to be greater than 10(-4) M. This result does not support the suggestion of Elton et al. [Elton, T. S., Dion, L.D., Bost, K. L., Oparil, S. & Blalock, J. E. (1988) Proc. Natl. Acad. Sci. USA 85, 2518-2522] that AII and IIA engage in high-affinity binding (Kd approximately 5 x 10(-8) M) with each other.