Dibutylhexamethylenediamine (DBHMD), a polymer intermediate, is strongly alkaline and is corrosive to eyes and skin. Its inhalation toxicity was defined in rats in both acute and subchronic studies. A 4-hr LC50 of 0.22 mg/litre was obtained for DBHMD of either 80.6 or 98.25% purity. Clinical signs of irritation were apparent during and immediately following exposure and no delayed deaths occurred. The mortality dose-response line was relatively steep. Reduction of exposure times to 5 min produced an LC50 of 51 mg/litre, with similar clinical signs and a steep dose-response line. These data suggest that the product of concentration and time (Ct) is constant for exposure times ranging from 5 min to 4 hr. Repeated (ten) exposures to 0.0125 mg/litre caused no signs of adverse response in rats. A concentration of 0.0234 mg/litre caused some mortality, mucous-membrane irritation, changes in haematological parameters (including erythrocyte counts, haemoglobin, concentration and total and differential leucocyte counts) and an increased acidity and decreased volume of urine. Death at this exposure level was attributed to cardiac failure secondary to pulmonary oedema and congestion, with evidence of hepatic congestion, diffuse cardiac myocytolysis and oedema, and thymic atrophy and congestion. Changes in the heart and thymus were seen in one of two rats killed after the tenth exposure. Clinical and pathological changes observed after the ten exposures were absent after a 14-day recovery period. DBHMD is highly toxic following acute inhalation and produces a steep dose-response following either single or multiple exposures.