The aim of the study was to identify the effects on the lung after subchronic glass fiber intratracheal instillation study on rats. We evaluated the toxicological effects on the lung: persistent inflammatory reaction, cell proliferation, and pulmonary fibrosis on histopathological examination. We performed a glass fiber intratracheal instillation study on total 32 Wistar rats. The animals were divided into four groups: three test groups exposed to different doses of glass fiber and one control group. One week after the end of the exposure period, all animals were euthanized. The histopathological examination of the lung performed in this study followed both distribution of the lesions through the multilevel biopsies that were taken and the inflammatory profile using both hematoxilin-eozin and Sirius red staining. The inflammatory lesions described for the first group were minimal/slight (grade I) and the total score was between 0 and 10 points (mean value = 3). For the second group, the inflammatory lesions were moderate/marked (grade II) with discrete collagen proliferation and discrete fibrosis and the total score ranged between 11 and 20 points (mean value = 11,250). For the third group, the described inflammatory lesions were massive with total score ranging between 21 and 30 points with collagen deposition, pulmonary and pleural fibrosis, and lung emphysema (mean value = 21,750) and no lesion in control group (with statistically significant difference P ≤ .001). This study of fiber glass intratracheal instillation of three different doses demonstrates that exposure to fiber glass is responsible for the development of persistent inflammatory response and a large range of hystopathological lesions which correlate to the administered dose.