The pharmacokinetic parameters disposition half-life, metabolic clearance, volume of distribution, intrinsic clearance of unbound drug, and (distributive tissue volume/unbound fraction in tissue) were compared for 12 benzodiazepines in dog and man. With the exception of volume of distribution, statistically significant correlations were obtained when parameters were plotted on a double logarithmic grid. In general, benzodiazepines were metabolized more rapidly and exhibited greater tissue distribution in dog than in man. The variability in parameters was such, however, as to make extrapolations from one species to another subject to considerable error.