The structure of the serpin, plasminogen activator inhibitor type-2 (PAI-2), in a complex with a peptide mimicking its reactive center loop (RCL) has been determined at 1.6-Å resolution. The structure shows the relaxed state serpin structure with a prominent six-stranded β-sheet. Clear electron density is seen for all residues in the peptide. The P1 residue of the peptide binds to a well defined pocket at the base of PAI-2 that may be important in determining the specificity of protease inhibition. The stressed-to-relaxed state (S → R) transition in PAI-2 can be modeled as the relative motion between a quasirigid core domain and a smaller segment comprising helix hF and β-strands s1A, s2A, and s3A. A comparison of the Ramachandran plots of the stressed and relaxed state PAI-2 structures reveals the location of several hinge regions connecting these two domains. The hinge regions cluster in three locations on the structure, ensuring a cooperative S → R transition. We hypothesize that the hinge formed by the conserved Gly206 on β-strand s3A in the breach region of PAI-2 effects the S → R transition by altering its backbone torsion angles. This torsional change is due to the binding of the P14 threonine of the RCL to the open breach region of PAI-2.