Hexamethyleneimine (CAS No. 111-49-9) was evaluated for acute inhalation toxicity in 6 male ChR-CD rats administered whole-body exposures to dynamically-generated vapor concentrations of 0.52, 1.32, 1.95, 2.45, 2.77, and 3.12 mg/L for 4 hours. During 14-day post-exposure observation, clinical signs of toxicity included chewing and grooming motions (0.52-1.95 mg/L), labored breathing (1.32-2.77 mg/L), gasping and redness about eyes and nose (2.45 mg/L), fasciculations (2.77, 3.12 mg/L), convulsions (2.77 mg/L), and death (exposures of 2.45 mg/L and above). All exposure groups exhibited at least a transient weight loss in study survivors, the duration, severity, and incidence exposure related. Mortality was consistent with a 4- hour inhalation LC50 in rats of approximately 2.45 mg/L. Upon necropsy, corneal opacity (exposures of 1.95-2.77 mg/L), lung congestion (1.95-2.77 mg/L), nasal discharge (2.45, 2.77 mg/L), and lacrimation (2.77 mg/L) were associated with treatment in both survivors and lethalities; all study lethalities exhibited mild to moderate lung pathology. Histopathologic examination of major organs and tissues in deceased rats only revealed potential toxic effects of lungs, trachea, and eyes.