The acute toxicity studies of taltirelin tetrahydrate (TA-0910), a new thyrotropin-releasing hormone (TRH) analogue, were performed in Slc:ddY mice, Slc:Wistar rats and beagle dogs of both sexes. The drug was administered to mice and rats by oral (p.o.), intravenous (i.v.) and subcutaneous (s.c.) routes, and to dogs by the p.o. and i.v. routes. LD50 values were more than 5,000 mg/kg in mice and rats of both sexes by the p.o. and s.c. routes. Some mice and rats died immediately after i.v. injection, the LD50 values were more than 2,000 mg/kg in mice of both sexes and calculated as 799 and 946 mg/kg in male and female rats, respectively. The minimum lethal doses were more than 2000 mg/kg in dogs of both sexes by the p.o. route. Though all dogs treated intravenously with 1000 mg/kg could survive during the observation period, a female dog with 500 mg/kg died on the day after administration. In general condition, hyperactivity, tremor and straub tail, that reflected central stimulatory effects of TA-0910, were observed in mice and rats, and also wet dog shaking in only rats. Vomiting and hyperactivity were seen in dogs by the p.o. route, and exaltation (during the dosing) and sedation by the i.v. route. In addition, salivation and transient tachycardia were observed in the both routes. In blood chemical examination, the transient changes of glucose, protein, lipid and/or serum enzyme were shown. In autopsy, no notable changes were seen in mice, rats and dogs.