[en] The most difficult radionuclides to measure by extemal chest counting are the isotopes of plutonium (Pu). They are detected through weak emission of low energy L X-rays. The Pu treated in nuclear fuel cycle is usually accompanied with ²⁴¹Am produced from ²⁴¹Pu by β disintegration, which emits γ-rays of 60 keV with the emission rate of 0.36, being more penetrable than the L X-rays. The ²⁴¹Am could improve the detection limit of chest counting of Pu, if it being used as a metabolic tracer for Pu in lungs. It has already been shown that the ²⁴¹Am which would probably be embedded in a matrix of PuO₂ is cleared from the lungs at the same rate as the Pu for long time after intake (N. Ishigure, et al., Radiat, Prot. Dosim., 79, 133, 1998), which supports the validity of ²⁴¹Am as a tracer of inhaled Pu in external chest counting. In the present work another possible chemical form present in work places, Pu nitrate, has been investigated. The solution of Pu(NO₃)₄ was nebulized using a compressed-air operated nebulizer. The resultant droplets were passed through a tube with fresh air and conduced into a multi-port nose-only exposure chamber, which resulted in polydisperse aerosols of Pu(NO₃)₄ with 0.6 μm in AMAD. Young adult male Wistar rats, being 13 weeks old and weighing 230 g at the time of exposure were used. The exposed rats were periodically sacrificed and the radioactivity of ²⁴¹Am and ^238/239/240Pu in the autopsied lungs were measured by photon spectrometry with a NaI/CsI phoswich detector system. The activity ratio of ²⁴¹Am vs. Pu in lungs, 2.4% at the exposure, slowly decreased to 2.1% at 4 week and 2.0% at 24 weeks. However, from practical point of view, it could be concluded that ²⁴¹Am would be a valid metabolic tracer for inhaled Pu nitrate at least for half a year post inhalation, considering the detection limit for Pu in chest counting, which is much greater than the ALI of Pu and the large uncertainties due to the estimation of chest wall thickness, the change in the distribution of particles within the lungs etc. (author)