Ten male and ten female Wistar rats per group were used in a subacute 4-week inhalation study with aerosolized RFR 910 (nebulization of the isocyanate in acetone as the vehicle; exposure - 6 hr/day, 5 days per week for four consecutive weeks) to mean actual concentrations of 0.16, 0.52, 2.1, and 10.4 mg of RFR 910/m3 of air under dynamic directed-flow nose-only exposure. Rats exposed to the vehicle (acetone), under identical test conditions, served as the control group. Concentrations up to and including 0.52 mg/m3 of air were well tolerated by all rats without test substance induced effects or mortality. In the 2.1 mg/m3 of air group, a few rats expedenced mild and transient signs of respiratory tract irritation. In the high-level exposure group, the rats experienced transiently decreased body weight (first exposure week only). The signs observed in this group were predominantly related to respiratory distress which appeared to attenuate during the course of the study. The clinical signs were: bradypnea, labored and unregular breathing pattem, piloerection, serous nasal discharge, and nostrils with red encrustations.