In rat liver plasma membrane a single angiotensin II (Ang II) binding site (K d of 3.71 ± 0.33 nM and B max of 1143.7 ± 83.9 fmol/mg protein) was identified using radioligand binding assay. Pharmacologically, this receptor match with the AT 1 receptor subtypes in term of affinity for the selective antagonist Losartan, and probably with the AT 1A receptor form in term of insensitivity for the antagonist PD123319. Nevertheless, using polyacrylamide gel isoelectric focusing, two 125I-Ang II binding sites migrating to pI 6.8 and 6.5 were found in these membrane preparations. Monophasic displacement of 125I-Ang II bound to isoform migrating at pI 6.8 clearly indicate that this isoform represents a functional Ang II-receptor complex. In contrast, the high concentrations of agonist and peptidic derivates of Ang necessary to displace 125I-Ang II bound to isoform migrating at pI 6.5 indicate that this atypical 125I-Ang II binding site represents a biologically nonfunctional Ang II binding molecule, presumably a nonspecific 125I-Ang II binding site.