Cetylpyridinium chloride (CPC) is a quaternary ammonium salt and cationic surfactant. It has been used as a biocide in personal hygiene products and a charge control additive in some reprographic toners. CPC is orally toxic to rats, mice and rabbits and can cause severe eye irritation. Acute inhalation toxicity studies of CPC and other quaternary ammonium salts have not, however, been reported. Groups of five rats per sex were exposed to aerosols containing 0 (control), 0.05, 0.07, 0.13 and 0.29 mg CPC/litre for 4 hr and observed for toxicity and ocular effects for 14 days thereafter. All animals were subjected to autopsy and the eyes were examined microscopically. The LC50 (sexes combined) of CPC was 0.09 mg/litre with upper and lower 95% confidence limits of 0.13 and 0.07 mg/litre, respectively. Clinical signs of toxicity included weight loss, nasal discharge, chromodacryorrhoea, respiratory difficulty and eye irritation, and all these non-lethal effects were reversible. Acute inflammation of the cornea, iris and/or aqueous humour were found in one, seven and four of 10 rats exposed to 0.07, 0.13 and 0.29 mg CPC/litre, respectively. Corneal epithelial hyalinization, without evidence of ongoing inflammation, was found in three additional rats among the 10 exposed to 0.29 mg CPC/litre.