The plutonium (Pu) treated in the nuclear fuel cycle is usually accompanied by 241Am produced from 241Pu by beta disintegration, which emits gamma rays of 60 keV with the emission rate of 0.36, being more penetrating than the L X rays from Pu. The 241Am could imprve the detection limit of chest counting of Pu, if it was being used as a metabolic tracer for Pu in lungs. Young adult male Wistar rats were exposed to polydisperse aerosols of Pu(NO3)4 with 0.6 microm in activity median aerodynamic diameter. They were periodically killed and the radioactivity of 241Am was cleared from the lungs at almost the same rate as the Pu at least for half a year post-inhalation, which proved the validity of 241Am as a metabolic tracer for inhaled Pu nitrate in external chest counting.