Changes in the distribution of MWCNTs in the lungs after inhalation exposure to MWCNTs. Changes in the lung burden are given for total lung burden, the airway region and the alveolar region. Burden in the alveolar region is further subdivided into that present in alveolar macrophages and that in alveolar tissue (including MWCNTs in subpleural alveolar tissue). Between day 1 and 14 days post-exposure the burden in the airway region rapidly declined to a near steady level for the remainder of the post-exposure period. The burden in the alveolar region paralleled the total lung burden with the small difference being due to that present in the airway region. Burden in alveolar macrophages was initially 3 fold higher than burden in the alveolar tissue but was nearly equal by 336 days post-exposure as the burden in alveolar macrophages declined and the burden in alveolar tissue increased. Data Mean ± S.E., N = 5.