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2-Cyclohexene-1-one (CHX) is a cyclic alpha,beta-unsaturated ketone with broad human exposure. CHX is an environmental pollutant and is present in tobacco smoke and in soft drinks sweetened with cyclamate. Interest in the toxicity of this class of compounds is due to their structural similarity to the cytotoxin acrolein. In a pilot study, rats and mice were exposed to 0, 20, 40, or 80 ppm CHX for 6 h/day. The study was terminated after 4 days due to acute toxicity in the high-dose groups. In a subsequent 14-day study, mice and rats were exposed to 0, 2.5, 5, or 10 ppm CHX for 6 h/day. All animals survived exposure until terminal sacrifice. Body weights were not significantly different from controls after 14 days of exposure. Liver/body weights were increased in male and female mice exposed to 5 and 10 ppm, and in male and female rats exposed to 10 ppm CHX. Ninety-day toxicity studies were conducted to provide data required to design chronic toxicity and carcinogenicity studies of CHX if it is determined such studies are necessary. Groups of 10 male and female F-344 rats and B6C3F1 mice were exposed to 0, 2.5, 5, or 10 ppm CHX for 6 h/day for 13 wk. All animals survived until sacrifice. Body weights were not significantly different from controls after 13 wk of exposure. Liver weights were increased in male and female mice exposed to 5 and 10 ppm and in male and female rats exposed to 10 ppm CHX. No adverse effects on bone-marrow micronuclei, sperm motility, or vaginal cytology were observed. Microscopic lesions included hyperplasia, and squamous metaplasia in the nasal cavity in rats and mice of both sexes at all doses. Nasal-cavity erosion and suppurative inflammation also occurred in high-dose mice. Larynx and lung were not affected in either sex or species. Dose-related hepatic centrilobular cytoplasmic vacuolation was seen in male rats only. These data suggest that CHX acts as an alkylating agent primarily producing toxicity at the exposure site.