PAI-1 is a proteinase inhibitor, which plays a key role in the regulation of fibrinolysis. It belongs to the serpins, a family of proteins that behave either as proteinase inhibitors or proteinase substrates, both reactions involving limited proteolysis of the reactive center loop and insertion of part of this loop into beta-sheet A. Titration calorimetry shows that the inhibition of tissue-type plasminogen and pancreatic trypsin are exothermic reactions with DeltaH = -20.3, and -22.5 kcal.mol(-1), respectively. The Pseudomonas aeruginosa elastase-catalyzed reactive center loop cleavage and inactivation of the inhibitor is also exothermic (DeltaH = -38.9 kcal.mol(-1)). The bacterial elastase also hydrolyses peptide-bound PAI-1 in which acetyl-TVASSSTA, the octapeptide corresponding to the P(14)-P(7) sequence of the reactive center loop is inserted into beta-sheet A of the serpin with DeltaH = -4.0 kcal.mol(-1). In contrast, DeltaH = 0 for the spontaneous conversion of the metastable active PAI-1 molecule into its thermodynamically stable inactive (latent) conformer although this conversion also involves loop/sheet insertion. We conclude that the active to latent transition of PAI-1 is an entirely entropy-driven phenomenon.