Acute oral toxicity was evaluated in groups of 5 male and 5 female young adult Charles River CD rats administered single doses of undiluted 1,2,4-trimethylbenzene by oral gavage at levels of 1470, 2150, 3160, 4640, 6810 and 10,000 mg/kg of body weight. Mortality was observed in 2 females and 1 male in the 3160 mg/kg dose group, and all 10 animals in each of the three highest dose groups. The LD50 value was calculated to be 3550 mg/kg (3040-4130 mg/kg, 95% confidence limits) for males and 3280 mg/kg (2720-3960 mg/kg 95% confidence limits) for females by the method of Horn. Clinical observations included inactivity, salivation, rough coat, piloerection, incoordination, arched back, Straub tail, tremors, eye and nasal discharge and prostration. Gross necropsy revealed dilated renal pelvis, dark lungs, white solid material in bladder, and discolored and friable liver in survivors, and bright red, dark, mottled lungs, distended urinary bladders, yellow mucous in the GI tract, prominent and engorged mesenteric vessels, pale mottled livers, and dark spots on the thymus in decedents.