AbstractCl‐966 (1‐[2‐[bis[4‐(trifluoromethyl)phenyl]methoxy]]‐1,2,5,6‐tetr5ahydro‐3‐pyridinecarboxylic acid, HCl salt) is an anticonvulsant agent that blocks the re‐uptake of gammaaminobutyric acid (GABA) into presynaptic nerve terminals and glial cells. The preclinical toxicology of Cl‐996 was evaluated in male and female B6C3F1 mice, Wistar rats, and beagle dogs. Doses were administered orally and are expressed as free acid equivalents. Acute (gavage) toxicity was characterized principally by CNS signs (hypoactivity, atasia, tremors, convulsions, and prostration); diarrhea and urine scald; and delayed mortality (2–3 days in mice and 2–11 days in rats) with 14 day median lethal doses of 653 and 825 mg/kg in male and female mice and 1,019 and 830 mg/kg in male and female rats, respectively. In a 4‐week rat study in which Cl‐966 was given in the diet, the high dose of 500 mg/kg/day was not tolerated, with marked decreases in body weight and food consumption necessitating sacrifice during the second week. The 150 mg/kg/day dose caused reduced body weight gain, lower food consumption, and hyperactivity, while 50 mg/kg/day was clearly a no‐effect level. In dogs, Cl‐966 was given in gelatin capsules at doses of 2 to 25mg/kg/day in an exploratory study and at 2,6, and 15 mg/kg/day in a 4‐week study. Prostration, ataxia, disorientation, and other mainly CNS‐related clinical signs occurred at 5 to 25mg/kg. Aspartate and/or alanine aminotransferase activities were elevated at 20 and 25 mg/kg in the exploratory dog study. However, there were no histopathologic findings in either species directly attributable to Cl‐966 treatment. Enhanced GABAergic activity was considered to be the cause of most of the clinical signs elicited by Cl‐966 in rodents and dogs. Blood Cl‐966 levels were dose‐dependent, but higher in females than males for both rats and dogs. These results were used to support initial clinical trials