The ability to predict the mechanisms and the associated rate constants of protein-ligand unbinding is of great practical importance in drug design. In this work we demonstrate how a recently introduced metadynamics-based approach allows exploration of the unbinding pathways, estimation of the rates, and determination of the rate-limiting steps in the paradigmatic case of the trypsin-benzamidine system. Protein, ligand, and solvent are described with full atomic resolution. Using metadynamics, multiple unbinding trajectories that start with the ligand in the crystallographic binding pose and end with the ligand in the fully solvated state are generated. The unbinding rate k off is computed from the mean residence time of the ligand. Using our previously computed binding affinity we also obtain the binding rate k on . Both rates are in agreement with reported experimental values. We uncover the complex pathways of unbinding trajectories and describe the critical ratelimiting steps with unprecedented detail. Our findings illuminate the role played by the coupling between subtle protein backbone fluctuations and the solvation by water molecules that enter the binding pocket and assist in the breaking of the shielded hydrogen bonds. We expect our approach to be useful in calculating rates for general protein-ligand systems and a valid support for drug design.protein-ligand unbinding | kinetics | enhanced sampling | drug design U nderstanding the thermodynamics and kinetics of proteinligand interactions is of paramount relevance in the early stages of drug discovery (1-3). So far the major emphasis has been placed on predicting the most likely binding pose as determined by the highest binding affinity (4, 5). In contrast, it has not been possible to predict the pathways for unbinding and the associated rates. However, it is by now well-recognized that one of the most pertinent factors for sustained drug efficacy and safety is not just its affinity, but possibly even more so, the mean lifetime of the protein-ligand complex (1-3). The latter property is strictly related to the time during which the ligand remains in the binding site (1, 2), and is typically expressed by its inverse, the dissociation rate k off (2). In principle k off should be amenable to calculations through all-atom molecular dynamics (MD) simulations. These simulations could give detailed and useful insights into the atomic interactions at work during unbinding, especially in the ephemeral but kinetically most relevant transition state ensemble (TSE) (6, 7). Such information is of great value in designing modifications of the ligand that might improve its pharmaceutical properties.However, despite the potential of MD simulations no such calculation has yet been reported. This is a consequence of the limited timescales of MD simulations. Even with the most modern purpose-built supercomputers or massive distributed computing, one can barely reach the timescale of milliseconds (3). Unfortunately most of the reported ligand-protein dissociation times fa...