Acute inhalation sensory irritation was evaluated in groups of 4 male Sprague-Dawley rats exposed to 2,4-toluene diisocyanate at analytical concentrations of 0.29, 0.88, 1.41 or 3.20 ppm for 3 hours. The test atmosphere was generated in a glass bubbler (immersed in a water bath at 25 deg C) through which varying rates of airflow were passed. Mortality was not observed. The RD50 value (concentration estimated to cause a 50% decrease in respiratory rate) was calculated to be 1.37 ppm (0.93-2.30 ppm 95% confidence limits) by probit analysis. Clinical observations included lacrimation, rhinorrhea, weight loss, labored breathing, red swollen conjunctiva, and rales. Gross necropsy revealed pulmonary lesions in the highest dose group animals.