Morphometric determination of the time-course of collagen response day 1 to day 56 post-exposure to an 80 ug MWCNT dose. Results from morphometric determination of Sirius Red stained alveolar interstitial collagen fibers expressed as the average thickness of connective tissue in the alveolar interstitium at various times after aspiration of MWCNTs. At 28 days, there was significant accumulation of connective tissue fibers in the alveolar wall which continued to increase through 56 days after aspiration. Day 1 and day 7 post-exposure were not significantly different from PBS-treated controls. Data are means ± SE for an N of 8 animals per group. * indicates statistical significance P < 0.05 vs. DM -treated control mice.