Five male and 5 female beagle dogs were orally given lactitol, a hepatic encephalopathy drug, for 52 weeks at doses of 0, 0.25, 1.25 or 6.25 g/kg/day. A 9 week recovery test was conducted after the discontinuation of the drug treatment. Soft stool, diarrhea, and vomiting were seen in the 1.25 and 6.25 g/kg groups. In the 6.25 g/kg group, bloody stool and increased water consumption were also observed. Urinalysis showed larger amount of the urine volume in the 6.25 g/kg group. The cecum weight of this group was increased without any morphological changes. There were no drug related effects on survival, body weight gain and food consumption. Electrocardiographic, ophthalmoscopic, hematologic and biochemical examinations failed to show any abnormalities related to the drug treatment. The above mentioned changes were satisfactorily reversible. Based on the results obtained, the NOAEL of this study was suggested to be 0.25 g/kg/day.