This study was designed to evaluate the distribution of mast cells in the lung tissues of rats exposed to biomass smoke. Fifty six female Wistar albino adult rats were used. They were divided into two experimental groups (control and biomass smoke-treated), each containing 28 animals. Control rats were not exposed to the biomass smoke at any time during the experiment. Rats in the treatment group were exposed daily (one hour) to biomass smoke for 3, 6 or 9 months. Lung tissues samples were obtained under deep anesthesia from the randomly selected 7 animals in both groups. Lung tissues were fixed in Mota's fixative (BLA) for 24 h and embedded in paraffin. Sections of 6 put thickness were cut and stained with 0.5% toluidine blue in 0.5 N hydrochloric acid at pH 0.5 for 30 min. The numbers of mast cell in lung tissues of the animals exposed to the biomass for 6 or 9 months were significantly (P<0.05) higher than controls. This study showed that long term exposure to biomass smoke was associated with the increased number of mast cells in the lung.