Correlations of lethal doses of industrial chemicals between oral or intraperitoneal administration and inhalation exposure in rats and mice were investigated. LC50 values for inhalation exposure and LD50 values for oral and intraperitoneal administration were obtained from the Registry of Toxic Effects of Chemical Substances. LC50 and LD50 values were plotted on ordinate and abscissa, respectively, using logarithmic scales. A correlation coefficient of r = 0.624 (n = 146, p < 0.001) was obtained for LC50 (ppm) and LD50 (mg/kg) values with oral administration (oral LD50) in rats. This correlation was improved by converting the units of LC50 from ppm to ppm h (cumulative dose), and by converting the units of LD50 from mg/kg to mmol/kg. The correlation coefficient was r = 0.742 when ppm*hr and mmol/kg were adopted for LC50 and LD50, respectively. A similar improvement in correlation coefficients by the same unit conversion was also observed between LC50 and LD50 with intraperitoneal (i.p.) administration (i.p. LD50) in rats. Correlations between LC50 and oral LD50 in mice were also improved by the same unit conversion. The correlations between LC50 and i.p. LD50 were higher than those between LC50 and oral LD50 both in rats and mice. In these correlations, coefficients obtained in rats were greater than corresponding coefficients in mice. We calculated equations to estimate LC50 values accompanied by confidence limits from oral or i.p. LD50 values.