This study was designed to evaluate that a difference in the route of absorption or the mode of lethality is responsible for the higher inhalation lethality of micron-sized organophosphorus insecticide, chlorfenvinphos (CVP), aerosols than the submicron-sized aerosols. Male Fischer 344 rats were exposed to the micron-sized (> 1 micron) or the submicron-sized aerosols (< 1 micron) for 4 h using the nose-only exposure system. LC50 of the micron-sized and the submicron-sized aerosols was 0.13 mg/l and 0.51 mg/l, respectively. Placing a drain cannula in the esophagus markedly increased LC50 of the micron-sized aerosols to 0.49 mg/l, but not that of the submicron-sized aerosols. There was no qualitative difference in lethal profile in cardiorespiration between 2 types of aerosols. The higher lethality of the micron-sized aerosols could be ascribed to swallowed CVP.