The toxicity of many volatile, industrial chemicals is due to the formation of toxic metabolites rather than the intrinsic toxicity of the chemicals themselves. Complete toxicological evaluations of such chemicals necessitate determination of the pertinent kinetic constants of the activation reaction. Previously, the studies required to determine these constants have been technically very difficult and time consuming. A much simpler technique to assess the kinetic constants for metabolism of gaseous toxicants has been developed. The rate of gas uptake by rats has been studied in a closed chamber system and by analyzing the time course of uptake curves obtained at various concentrations the K m (as atmospheric ppm) of the reaction as well as the maximal rate of metabolism, V max, was determined. Using these inhalation techniques, the metabolism of 1,1-dichloroethylene (1,1-DCE) has been examined in detail. Gas phase uptake of 1,1-DCE occurred in two distinct phases. A rapid phase with a concentration-independent rate constant of 2.2 hr −1 represented whole body equilibration. The magnitude of the rapid phase was proportional to the mass of rats in the chamber and the concentration of 1,1-DCE. Rapid phase uptake corresponded to a whole body/gas distribution coefficient of 4.04. The slow phase represented metabolism and was abolished by pretreatment with pyrazole or simultaneous exposure to CCl 4. The concentration dependence of the rate of the slow phase followed the Michealis-Menten form with an inhalation K m of 335 ppm and a chamber V max of 132 ppm/kg/hr (corresponding to an in vivo V max of 15.87 mg metabolized/kg/hr). From these rate data the acute inhalation toxicity in naive rats was shown to be a direct function of the amount of metabolite formed, and not of the concentration of 1,1-DCE. Short term exposures produced mortality after formation of 25–30 mg of metabolite/kg. At concentrations between 200–1000 ppm, a direct correspondence was found between the time of exposure required to kill rats and the time calculated to produce a constant amount of metabolite(s).