Male and female Sprague-Dawley derived (CD) rats were exposed to hexamethylene diamine aerosols for six hours per day, five days per week for thirteen weeks. The target concentrations were 0.0, 12.5, 50.0 and 200.0 milligrams per cubic meter (mg/m3). Hematology and clinical chemistry para- meters were examined prior to exposure, at Week 5 and at the termination of the exposure period (Week 13). Group IV was terminated at Week 7 because of high mortality after six weeks of exposure to the test material. Exposure to the test material caused signs of respiratory and ocular .irritation at the 50.0 and 200.0 mg/m3 levels. The animals' fur became discolored and contaminated with the test material and was most prevalent at the high level. The animals in the 200 mg/m3 group displayed respiratory dysfunction, yellow ano-genital staining general debilitation, weight loss and poor condition prior to death or sacrifice. Body weight gains were reduced throughout the study in both sexes receiving 200 mg/m3 and females at the intermediate level; males at the intermediate level were similarly affected in Weeks 1-8. The body weights in the group receiving 12.5 mg/m3 were similar to the control group and did not indicate a response to treatment. At the 13 Neek examination the glucose levels in the females at the low and intermediate levels were significantly depressed and appeared to be dose related. All other chemical constituents were not effected by the 13 Week exposure to 12.5 or 50.0 mg/m3 of hexamethylene diamine. Squamous metaplasia of tracheal and nasal epithelia, acute rhinitis and interstitial pneumonia were seen at the high level; no treatment-related micropathology was seen at the intermediate level.