In this study, we have evaluated the pulmonary toxicity of intratracheally (i.t.) instilled two multi walled carbon nanotubes (MWCNT) in rats. The lungs of rats were instilled with phosphate buffered saline + 1% of Tween 80 or MWCNT or carbonyl iron or quartz particles at a dose of 0.2, 1, and 5 mg/kg b.w. Following exposure, bronchoalveolar lavage (BAL) fluid was collected from the lungs to analyze lactate dehydrogenase (LDH), alkaline phosphatase (ALP), lipid peroxidation products (MDA; malondialdehyde), and total microprotein (MTP) levels at 24 h, one week, one month, and three months post instillation periods. The lungs of particle exposed rats were also collected at the same intervals to evaluate for histopathology. Exposures of MWCNT and quartz particles to rats produced transient dose dependant increase in BAL fluid LDH, ALP, MDA, and MTP values than control at all post exposure periods. Results of lung histopathology revealed that exposure of MWCNT produced a dose dependant focal peribronchiolar lymphoid aggregates, foamy alveolar macrophage accumulation, lymphoplasmocytic infiltration, fibrosis and diffuse alveolar damage. In conclusion, instillation of MWCNT produced a greater pulmonary toxicity in rats and was comparable with that of quartz. © 2010 Wiley Periodicals, Inc. Environ Toxicol, 2012.