Protease inhibition by serpins requires a large conformational transition from an active, metastable state to an inactive, stable state. Similar reactions can also occur in the absence of proteases, and these latency transitions take hours, making their time scales many orders of magnitude larger than are currently accessible using conventional molecular dynamics simulations. Using a variational path sampling algorithm, we simulated the entire serpin active-to-latent transition in all-atom detail with a physically realistic force field using a standard computing cluster. These simulations provide a unifying picture explaining existing experimental data for the latency transition of the serpin plasminogen activator inhibitor-1 (PAI-1). They predict a long-lived intermediate that resembles a previously proposed, partially loop-inserted, prelatent state; correctly predict the effects of PAI-1 mutations on the kinetics; and provide a potential means to identify ligands able to accelerate the latency transition. Interestingly, although all of the simulated PAI-1 variants readily access the prelatent intermediate, this conformation is not populated in the active-to-latent transition of another serpin, α 1 -antitrypsin, which does not readily go latent. Thus, these simulations also help elucidate why some inhibitory serpin families are more conformationally labile than others. T he serpin plasminogen activator inhibitor 1 (PAI-1) negatively regulates blood clot clearance (fibrinolysis) by mechanically inhibiting important serine proteases, including tissue type plasminogen activator and urokinase type plasminogen activator (1). Suicide inhibition, initiated by proteolytic cleavage of the PAI-1 reactive center loop (RCL), requires insertion of the cleaved RCL into the central β-sheet (sheet A). This process expands sheet A, inhibits the covalently attached protease by mechanical disruption of the active site (2), and results in a thermodynamically stable serpin conformation. Alternatively, PAI-1 can spontaneously deactivate by inserting its intact, uncleaved RCL into sheet A, resulting in the more stable but inactive latent conformation (1) (Fig. 1).The latency transition provides a facile way to regulate PAI-1 activity. Physiologically, this regulation is achieved by binding to the cell adhesion factor vitronectin, leading to an ∼50% increase in the active state t 1/2 (3). Because high levels of active PAI-1 are associated both with poor prognoses for some cancers, presumably due to interactions with vitronectin, and with cardiovascular diseases (1), PAI-1 inhibitors that accelerate the latency transition are under development (1, 4, 5). However, drug design efforts are hampered by the lack of detailed molecular mechanisms for PAI-1 conformational changes. Numerous studies have identified mutations that either accelerate or retard the conformational transition, as well as antibodies that can accelerate latency. Despite these efforts, the molecular details of the latency transition and the residues involved in the key i...