Kaempferia parviflora (KP) is a plant widely used in Southeast Asia. Its major compounds are 3,5,7,3',4'-pentamethoxyflavone (PMF), 5,7,4'-trimethoxylflavone (TMF), and 5,7-dimethoxyflavone (DMF). This study investigated the effect of KP extract on the blood levels and pharmacokinetics of sildenafil co-administration in rats. Rats were randomly assigned to four groups. Groups 1, 2, and 3 were given sildenafil 20mg/kg daily for 9days. On days 4-9 of each treatment period, the treated rats received KP extract (250mg/kg) and vehicle (groups 2 and 3, respectively). Group 4 received KP extract only (250mg/kg daily for 9days). Daily blood concentrations of sildenafil, PMF, TMF, and DMF were determined by HPLC to evaluate the daily blood level interactions. Additional blood samples were collected at various times on the last day of treatment to evaluate the pharmacokinetic interactions. The KP extract decreased blood levels of sildenafil on the first day of co-administration by 95% but the percentage reduction was insignificant on subsequent days. When co-administered with KP extract, the area under the curve (AUC), maximum concentration (C max), and half-life (T 1/2) of sildenafil were decreased by 60-65, 40-52, and 32-54%, respectively, with the elimination rate constant (K e) increased by 37-77%. In addition, PMF, TMF, and DMF concentrations and their AUC, C max, T max, K e, and T 1/2 values were changed after co-administration of KP extract and sildenafil.