Disposition kinetic variables of HI-6, a bispyridinium oxime, have been determined in mice, rats, rabbits, Rhesus monkeys, Beagles, sheep, and human beings. The drug has a short half-life, small apparent volume of distribution and high body clearance in these species, and is eliminated mainly by renal excretion. Using regression analysis and double logarithmic plots of the pharmacokinetic variables vs body weight of the various species, it was observed that body (systemic) clearance is the pharmacokinetic variable to use for interspecies comparison of elimination of the drug. The allometric exponent denoting the proportionality of body clearance of HI-6 to body weight of the 7 species studied was 0.76, which may be related to the renal excretion process for the drug. The apparent volume of distribution was similar (260 to 304 ml/kg of body weight) in the various species. The results indicate that volume of distribution, body clearance, and with less confidence, half-life might be used for interspecies scaling and for predicting these variables in other mammalian species. On the basis of the pharmacokinetic variables in selected species (rats and mice excluded), i.v. administration of HI-6 at a dosing rate of 20 to 25 mg/kg at 4-hour intervals should provide an average steady-state plasma concentration of 16 to 20 micrograms/ml in domestic animals. The short half-life of HI-6 precludes increasing the dosage interval.