Estimating the carcinogenic potential of exposure to diesel-engine ehaust particulates (DEPs) is problematic. In rats, high concentrations of DEPs (> 1,000 pg/m3) inhaled over a lifetime result in excess lung tumors. However, data for rats exposed to DEP at concentrtion not associated with lung overload are consistent with no tumorigenic effect Individual rat studies have only a limited number of exposure groups; therefore, we combined the tumor data from eight chronic inhalation studies in a meta-analysis. (16)]. Therefore, the lung tumor response in heavily exposed laboratory rats is not believed to be relevant to humans (8,(17)(18)(19)(20)(21)(22), and the high-dose rats are not a reliable basis for DEP quantitative risk assessment.