A subchronic inhalation toxicity study was conducted with groups of male and female Sprague-Dawley rats receiving whole body exposure to acetone cyanohydrin at mean analytical concentrations of 0, 9.2, 29.9 or 59.6ppm in a dynamic air flow chamber. At each concentration, groups of 20 rats (10 male & 10 female) were exposed 6 hours per day, over approximately a 4 week period. Mid- and high dose groups experienced irritation of the eye and/or mouth and difficulties in breathing. Following the first exposure, four high dose males were observed with signs associated with anoxia/hypoxia, such as respiratory distress, tremors and/or convulsions, foaming at the mouth and prostrate posture. Three of these animals subsequently died. Total serum protein was decreased in male rats at all exposure levels, but only significant at the mid- and high exposures. A significant decrease in RBC, HGB, MCHC and LDH 1 in high dose females and a significant decrease in LDH in mid dose males were observed. BUN was elevated in high dose females and serum T3 was significantly increased in mid-dose males. Mid- and high dose males were observed with increased relative liver weights. Serum thiocyanate levels were significantly elevated in all exposed animals and urine thiocyanate was significantly elevated only in the mid- and high dose groups. No gross or microscopic lesions were detected which could be attributed to acetone cyanohydrin.