Brownian dynamics simulations of a wild type and mutants of bovine pancreatic trypsin inhibitors were performed using the program brownian. The results of the simulations were consistent with experimentally determined stabilities of these proteins, and the computational times of the simulations were much less than those of molecular dynamics simulations. These results indicate that Brownian dynamics simulations can be useful for investigating structural features of proteins that have some mutants, such as drug-metabolizing enzymes.