Acute inhalation toxicity was evaluated in 5 groups of 10 male Sprague-Dawley rats receiving a single application of benzotriazole at concentration levels of 0.78, 1.46, 2.03, 2.23, and 2.71 mg/L for 3 hours. Mortality was observed at all concentration levels. The LC50 was 1.91 mg/L (95% confidence limits = 1.59 to 2.29 mg/L). Clinical observations included deep abdominal breathing with open mouth gasping. Gross necropsy revealed a severe accumulation of white frothy fluid in the trachea and a moderate to severe incidence of dark red hemorrhagic areas in the lungs.