Toxic effects of exposure of 1,2,4-trimethylbenzene (pseudocumene) in the condition of sub-chronic inhalation experiment were examined. Rats were exposed to vapours of pseudocumene at concentrations of 123 mg/m3, 492 mg/m3 and 1230 mg/m3, 6 h/day, 5 days/week for 3 months. After 3 months of inhalation exposure animals were necropsied. Blood samples were obtained and selected organs were weighted and prepared for histological examinations. Sub-chronic inhalation exposure to pseudocumene resulted in an overall low degree of systemic toxicity. There were no changes in body weight gain, food consumption and absolute and relative organ weights. Slightly higher activity of sorbitol dehydrogenase was observed in male rats exposed to all concentrations applied. Some disturbances in hematological parameters characterised by decrease in red and increase in white blood cells were observed in male rats exposed to high concentration of 1230 mg/m3. The pulmonary lesions observed in male and female rats were statistically significant at mid and high concentrations of pseudocumene.