Metformin is one of the most common medicines for the treatment of type 2 diabetes, however, recent studies suggest that concomitant antihyperglycemic agents should be administered for better efficacy. Yukmijihwang-tang (YMJHT) is a nephroprotective polyherb prescribed for renal disorders or diabetic mellitus in traditional Korean medicine. Therefore, the pharmacokinetics between metformin and YMJHT were examined for their coadministration. Rats were orally coadministered with metformin and YMJHT as a combination group or metformin and distilled water as the corresponding control. Then, the metformin concentration in plasma and its pharmacokinetic parameters including maximum concentration (Cmax) and area under the plasma concentration time curve (AUC) were analyzed. There were no interactions between metformin and YMJHT in the single coadministration at intervals within 5 min. However, pretreatments with YMJHT for 6 days increased the metformin concentration and its Cmax and AUC (p<0.05). The repeated coadministration for 8 days increased the Cmax of metformin (p<0.05). Conversely, when the combination was coadministered at 2h -intervals, there were no interactions between metformin and YMJHT after a single dosing or repeated dosing of coadministration for 7 days. These results of the present study will help structure proper dosing regimens for the concomitant therapy of metformin and YMJHT.