Aim: To investigate the relationship of alveolar macrophages and inhaled nanoparticles in the lung. Materials & methods: Rats were exposed by inhalation to 14 nm gold nanoparticles for 6 hours, and ultramicroscopic observation on the frequency and localization of gold nanoparticles within lavaged macrophages was performed up to 7 days. Results & discussion: The majority of macrophages examined on day 0 (94%) contained internalized gold nanoparticles, and the percentage decreased to 59% on day 7. Gold nanoparticles were exclusively found within cytoplasmic vesicles. On day 0, most gold nanoparticles appeared to be individual or slightly agglomerated, and they were frequently agglomerated on day 7. Conclusion: Alveolar macrophages efficiently internalized nanoparticles by endocytosis, and re-arrangements of vesicles and of nanoparticles in the vesicles of macrophages occurred.