Objectives: In this study, we investigated the oral toxicity of Gami-Jakyak Gamcho buja Decoction (Mecasin) to develop safe treatments. Methods: All experiments were conducted at the Medvill, an institution authorized to perform non-clinical studies, under the Good Laboratory Practice (GLP) regulations. In order to investigate the oral toxicity of Mecasin, we administered Mecasin orally to rats. Sprague-Dawley rats were divided into four groups of five male and five female animals per group: group 1 being the control group and groups 2, 3, and 4 being the experimental groups. Doses of Mecasin, 500 mg/kg, 1,000 mg/kg and 2,000 mg/kg, were administered to the experimental groups, and a dose of normal saline solution, 10 mL/kg, was administered to the control group. We examined the survival rate, weight, clinical signs, and gross findings. This study was conducted under the approval of the Institutional Animal Ethics Committee. Results: No deaths or abnormalities occurred in any of the four groups. Although slight decreases in the weights of some female rats were noted on the third day, no significant changes in weights or gross findings between the control group and the experimental groups were observed. To check for abnormalities in organs, we used microscopy to examine representative histological sections of each specified organ; the results showed no significant differences in any of the organs. Conclusion: The results showed that administration of 500 - 2,000 mg/kg of Mecasin did not cause any changes in weight or in the results of necropsy examinations. It also did not result in any mortalities. The above findings suggest that treatment with Mecasin is relatively safe. Further studies on this subject are needed to yield more concrete evidence.