Paeonia lactiflora Pall. (Ranunculaceae) root and Sinomenium acutum Rehder and Wilson (Menispermaceae) stem are two herbs widely used in Chinese medicine to treat rheumatoid arthritis. While, in theory, either herb could be used alone, in practice, Chinese medicine practitioners prescribe them together. Studies on pharmacokinetic interaction between the active constituents of these two herbs (paeoniflorin and sinomenine, respectively) provide empirical evidence to support their clinical practice. A single dose of paeoniflorin (150 mg/kg) alone and with sinomenine hydrochloride (90 mg/kg) was administered by gastric gavage to unrestrained conscious male Sprague-Dawley rats (n=5 or 6, 240-270 [corrected] g). Blood samples were collected periodically via a jugular vein before and after dosing from 10 min to 12 h. A high-performance liquid chromatographic (HPLC) assay was developed to determine the plasma concentrations of paeoniflorin. Non-compartmental pharmacokinetic profiles were constructed by using the software PK Solutions 2.0. The pharmacokinetic parameters were compared using unpaired Student t-test. After co-administration of sinomenine, the peak plasma concentration of paeoniflorin was elevated (P<0.01), the peak time was delayed (P<0.01), the AUC(0-t) was increased (P<0.001), the mean residence time (MRT) was prolonged (P<0.01), the C(L) was decreased (P<0.01) and the V(d) was reduced (P<0.05). These results indicate that sinomenine hydrochloride at 90 mg/kg significantly improved the bioavailability of paeoniflorin in rats.