Prion diseases, also known as transmissible spongiform encephalopathies, are fatal neurodegenerative disorders for which no effective curative or prophylactic method has been established. Recently, we discovered a novel antiprion compound, GN8. Administration of GN8 was found to prolong the survival of prion-infected mice. The aim of this study was to characterize the toxicological and pharmacological features of GN8 in rats and dogs treated via a single intravenous injection. Minimum lethal doses of GN8 were estimated to be approximately 60 and 40 mg/kg in rats and dogs, respectively. In the respiratory toxicity experiments, GN8 was administered to rats at doses of 0, 15.6, and 46.9 mg/kg, and rats were observed for consciousness, behavior, autonomic nervous symptoms, and body weights. GN8 was found to have little adverse effect on the rat respiratory system at a dose of 46.9 mg/kg. In the cardiovascular toxicity experiments, GN8 was administered to dogs at doses of 0, 7.8, and 31.3 mg/kg, and dogs were observed similarly. Although GN8 was found to have a slight effect on the cardiovascular system at a dose of 31.3 mg/kg, we did not find severe adverse effects of GN8 at doses sufficient for antiprion activity. This study would serve as a stepping stone to a clinical application of GN8 as an antiprion agent.