The spread of antibiotic-resistant bacteria in many humans and animals has driven researches to identify and design novel antibacterial agents. In vitro inhibitory activity of (2E)-2-(4,5-dihydro-4-oxothiazol-2-yl)-2-(thiazolidin-2-ylidene) acetonitrile against many bacterial pathogens has been proven in both veterinary and human medicine. In this study, its in vivo toxic effects was studied in mice. The median lethal dose (LD50) value of 239.88 mg/kg was estimated using intraperitoneal injection in 8 groups of mice after 48 h treatment. Then, intraperitoneal injections of LD50 of oxothiazole solution into 4 other mice were done to evaluate histopathological changes in their liver and kidney tissues. The histopathological studies were identified as fatty change, hepatitis, necrosis and regeneration in liver, and fibrosis, necrosis, nephritis, hyaline cast and hyperaemia in kidney. In conclusion, the synthesized oxothiazole derivative causes renal and hepatic toxicity in mice at medium concentrations. The change of thiazole substituents and complexation may reduce its toxicity