Microscopic examination of the tracheal dimensions of normal Florida mice (Peromyscus floridanus) and cotton mice (P. gossypinus) showed no significant differences between the two species, but external examination showed the tracheal length of the Florida mouse to be longer than that of the cotton mouse. Microscopic examination of the intrapulmonary apparatus (bronchioles, alveolar ducts, atria, and alveoli) of normal Florida and cotton mice showed no significant differences in measurements between the two species. Cotton mice were exposed to exhaust gases produced by the burning of solid rocket motor (SRM) fuel. Mice exposed once for a duration of 10 min demonstrated an LD50 of 52 to 56 ppm HCl/g body weight and an LD50 of 169 to 173 mg Al2O3/m3/g body weight. These LD50 values suggested that SRM exhaust components may have a synergistic lethal effect when compared to the effects of individual components of the exhaust. Cotton mice exposed to the exhaust exhibited external signs of respiratory distress and dyspnea. Those mice that received lethal exposures showed internal signs of early inflammatory reactions. However, the most likely cause of death was a sudden shift in blood pH.