A repeated dose toxicity study of montirelin hydrate (NS-3), a new drug for the treatment of disturbance of consciousness, was conducted in Sprague-Dawley rats. Male and female rats were given the drug intravenously for 5 weeks at doses of 0 (control), 0.05, 0.5, 5 and 50 mg/kg. After discontinuation of the treatment, a 4-week recovery test was also conducted in the 0, 0.5 and 50 mg/kg groups. No deaths related to the treatment were observed. Tremor was seen in the 50 mg/kg group. Polyuria was observed in the 5 and 50 mg/kg groups. There were decreases in body weight gain in the 0.5 mg/kg group and over of males, and in food consumption in all male dose groups and increase in water consumption in the 5 and 50 mg/kg groups. In blood chemical examination, decrease in triglyceride was observed in the 5 and 50 mg/kg groups of males. Urinalysis showed increase in urine volume in the 0.5 mg/kg group and over. Ophthalmoscopic and hematologic examinations failed to show any abnormalities attributable to the treatment. Pathological examination disclosed serous cell hypertrophy of the submandibular gland in all dose groups and increase in its organ weight in the 0.5 mg/kg group and over. The changes mentioned above were satisfactorily reversible except for the serous cell hypertrophy of the submandibular gland in the 50 mg/kg group. The decrease in food consumption and serous cell hypertrophy of the submandibular gland in the 0.05 mg/kg group were considered to be of no toxicological significance. These results show that the NOAEL of montirelin hydrate is 0.05 mg/kg for 5-week repeated dose toxicity in rats.