Summary: [Sar1,Phe(Br5)8] angiotensin II (Br5Ang II) is a specific, quasi-irreversible antagonist of angiotensin II (Ang II) in vitro. In vivo, this compound is a very potent, Ang II-specific antagonist with a very long duration of action against Ang II-induced blood pressure (BP) increases. In the “low-sodium” dog, this compound induces a prolonged BP reduction during and after intravenous infusion at doses comparable to cilazapril, a potent ACE inhibitor. The physicochemical and pharmacokinetic behavior of this peptide was therefore assessed to understand and interpret the prolonged antagonistic and antihypertensive activity of this peptide. Binding studies using beef adrenocortical membranes indicated specific binding of Br5Ang and related analogues to Ang II receptors with a Kd of 1.41 × 10-9 M against iodinated [Sar1, d-Phe8]Ang II, a standard radioligand antagonist. Iodinated Br5Ang II exhibited a very high degree of nonspecific binding to the membranes. It had an octanol-water partitioning coefficient of log P of + 0.903, a coefficient 84-fold higher than for [125I][Sar1,d-Phe8]Ang II. Association kinetics of [125I]Br5Ang II were similar to the standard ligand [125I] [Sar1,d-Phe8]Ang II, but the half-life of dissociation was four times higher (60 vs. 15 min at 20°C). Molecular modeling indicates a practically identical conformational behavior of both peptides, Br5Ang II and [Sar1,d-Phe8]Ang II but with an expanded hydration shell over the Br5 residue. It is concluded that the prolonged duration of action is due to the increased hydrophobicity of the peptide, which leads to a slow dissociation from the Ang II receptor. Furthermore, the pronounced partitioning effect into the membraneous phase may serve as a metabolically inert storage site for the biologically active peptide.