The present finding that mice metabolize a greater proportion of an oral dose (50 mg/kg) of vinylidence chloride. (1,1 - dichloroethylene, DCE) than rats implies (a) that the efficiency of DCE metabolism follows the known activity of cytochrome P-450 in the organs of these animals, and (b) that, in accordance with the LD(50) values, the real exposure (expressed as the amount of DCE metabolized) is relatively higher for orally dosed mice than rats, and (c) that DCE carcinogenicity would appear to be more likely in mice than rats.Mice metabolize DCE simiarly to rats (Jones and Hathway, 1977) but there are some differences. Thus, qualitatively, treated mice (but not rats) excrete a small amount of N-acetyl-S-(2carboxymethyl)cysteine. Quantitatively, (i) the relative proportions of the N-acetyl-S-(2-cysteinyl acetyl derivative that are formed in mice and rats parallel the activity of liver glutathione-S-epoxide transferase in these rodents, and (ii) there are marked differences in the proportions of DCE metabolites belonging to the chloroacetic acid branch of the metabolic pathway. Furthermore, the previously assumed β-thionase hydrolysis of thiodiglycollic acid (Jones and Hathway, 1977) is now established in vivo, and the possible biogenesis of the N-acetyl-S-cysteinyl acetyl derivative is verified by another tracer study. The conclusion is drawn that the DCE metabolites, 1,1-dichloroethylene oxide and chloroacetyl chloride, may be important to murine DCE carcinogenicity.