The acute toxicity of hexazinone, a herbicide intended for general noncropland areas and selected crop uses (alfalfa and sugarcane), has been evaluated to establish proper handling guidelines and to measure its potential impact on the environment. The material is slightly to moderately toxic when given as a single oral dose; its LD50 in male rats is 1690 mg/kg, in male guinea pigs 860 mg/kg, and in male dogs greater than 3400 mg/kg although in the dog emesis prevented accurate quantitation. When the material is administered intraperitoneally, the LD50 in rats is 530 mg/kg. Repeated doses (five oral doses per week for 2 weeks) of 300 mg/kg to rats produced slight weight loss in one of two replicate experiments. In both studies, no gross or histologic alterations were apparent. Hexazinone is a moderate to severe eye irritant in the rabbit and produced only mild erythema in rabbit skin at 5278 mg/kg, a dose which did not produce lethality or other clinical signs. Subchronic dermal exposures (10 consecutive doses) to rabbits produced increases in serum alkaline phosphatase and glutamic-pyruvic transaminase at the highest levels tested (680 and 770 mg/kg in two separate experiments) with no effects seen at 150 mg/kg. There were no alterations in livers from treated rabbits examined by light microscopy. No dermal sensitization was produced when concentrations of up to 50% were tested in guinea pigs. One-hour inhalation exposure of up to 7.48 mg/liter did not produce mortality in rats.(ABSTRACT TRUNCATED AT 250 WORDS)