Mitragyna speciosa Korth (MS) leaves are popularly used in Thailand and Malaysia. The leaves are used as a medicinal agent for treating diarrhea, worm infestations and acts as an analgesic and antipyretic agents. The present study investigated the acute toxicity of standardized MS leaf aqueous extract on Sprague Dawley rats at 175, 500 and 2000 mg/kg body weight. Dose selection was based on the Up-and-down procedure (OECD 425). No death was observed. The single oral dose of the extract also had no significant changes in the body weight, food and water consumption. The relative weights of the internal organs were also normal. However, the mean corpuscular hemoglobin concentration, albumin, calcium and cholesterol were significantly decreased. Histological examination showed steatosis and centrilobular necrosis only on several parts of the liver for the treated group. In short, the standardized MS aqueous extract did not cause any lethality and only caused slight toxic effects.