Metabolism of 1,3-butadiene to 1,2-epoxybutene-3 in rats follows saturation kinetics. Comparative investigation of inhalation pharmacokinetics in mice also revealed a saturation pattern. For both species "linear" pharmacokinetics apply at exposure concentrations below 1000 ppm 1,3-butadiene; saturation of butadiene metabolism is observed at atmospheric concentrations of about 2000 ppm. For mice metabolic clearance per kg body weight in the lower concentration range where first order metabolism applies was 7300 ml X h-1 (rat: 4500 ml X h-1. Maximal metabolic elimination rate (Vmax) was 400 mumol X h-1 X kg-1 (rat: 220 mumol X h-1 X kg-1. This shows that 1,3-butadiene is metabolized by mice at higher rates compared to rats. Based on these investigations, the metabolic elimination rates of butadiene in both species were calculated for the exposure concentrations applied in two inhalation bioassays with rats and with mice. The results show that the higher rate of butadiene metabolism in mice when compared to rats may only in part be responsible for the considerable difference in the susceptibility of both species to butadiene-induced carcinogenesis.