Translocation of inhaled ultrafine particles from the lungs into the blood may impair cardiovascular function. We administered ultrafine (20-nm) and fine (200-nm) gold colloid or fluorescein-labeled polystyrene particles to mice intratracheally and examined their localization in the lung and extrapulmonary organs. Fifteen minutes after instillation, dispersed and agglomerated 20-nm gold colloid particles were observed on the surface of endothelial cells, on the alveolar surface, in endocytotic vesicles of alveolar epithelial cells, and in the basement membrane of the lung. A small but noteworthy amount of gold was detected in the liver, kidney, spleen, and heart by inductively coupled plasma-mass spectrometry. After administration of 20- or 200-nm fluorescent particles, free particles were detected infrequently in blood vessels, on the endocardial surface, and in the kidney and liver only in the mice that received 20-nm particles, whereas phagocytes containing 20- or 200-nm particles were found in the extrapulmonary tissues. Fluorescent particle-laden alveolar macrophages administered intratracheally translocated from alveoli to extrapulmonary organs via the blood circulation. Thus, small amounts of ultrafine particles are transported across the alveolar wall into the blood circulation via endocytotic pathways, but particle-laden alveolar macrophages translocate both ultrafine and fine particles from the lungs to the extrapulmonary organs.