A 6-month repeated dose toxicity study with 1-month recovery test of sodium N-[2-[4-(2,2-dimethylpropionyloxy) phenylsulfonylamino] benzoyl] aminoacetate tetrahydrate (ONO-5046.Na), a novel neutrophil elastase inhibitor, was conducted in beagle dogs. The dogs of both sexes were administered ONO-5046.Na intravenously at a daily dose of 0 (vehicle control), 7.5, 15 or 30 mg/kg. In the 15 mg/kg and above groups, transient ataxic gait was observed. It is considered that this symptom could be attributed to the pharmacological effect of ONO-5046.Na. Macro- and microscopic hemorrhage at the injection site was observed in the ONO-5046.Na treated groups. However, it is considered that these findings could be attributed to the long-term repeated dosing procedure, and were not toxic changes. There were no treatment-related changes in body weight, food consumption, ophthalmology, urinalysis, hematology, blood chemistry, electrocardiography and organ weights. These results indicate that the NOAEL of ONO-5046.Na in dogs is 30 mg/kg/day for both sexes in this study.