Acute oral toxicity was evaluated in Sprague-Dawley rats administered p-tert-butyl benzoic acid using an up and down schedule. Mortality was observed in 1 out of 3 males receiving doses of 0.7 g/kg body weight, in 2 out of 2 males receiving doses of 0.9 g/kg, and in all 6 females receiving doses of 0.7 g/kg body weight. The LD50 value was determined to be 0.72 g/kg (95% confidence limits of 0.54 to 0.98 g/kg) in males; and was estimated to be less than 0.70 g/kg in females. Clinical observations included hypoactivity, ataxia, thin appearance, hunched posture, impaired use of front limbs, yellow-stained anal area, tremors, prostration, excess salivation, hypothermia, bradypnea, absence of pain reflex, spasticity, mydriasis, flaccidity, and respiratory congestion. One animal also exhibited cyanosis, absence of pain reflex, and a reddish material on its face and front paws. Gross necropsy of decedents and animals sacrificed at 7 days revealed: diffusely red glandular mucosa in stomachs; clear fluid in stomachs; enlarged stomach with clear red mucoid fluid; tan to dark tan mucoid material in intestines; red perinasal discharge; and enlarged right submandular lymph nodes.