The purpose of this study is to investigate the effect of aspirin on the pharmacokinetics of pranoprofen by oral coadministration of pranoprofen (5 mg/kg) with aspirin (5, 10 and 20 mg/kg) in Sprague-Dawley rats. After oral coadministration of pranoprofen with aspirin, the area under the plasma concentration-time curves (AUC) of pranoprofen was increased significantly by 10 mg/kg (p<0.05) and 20 mg/kg (p<0.01) of aspirin coadministration, and peak concentrations ($C_{max}$) of pranoprofen was increased significantly by coadministration of 20 mg/kg aspirin (p<0.05) compared to pranoprofen alone. Relative bioavailabilities (RB${\%}$) of pranoprofen in coadmistration were higher (from 1.42 to 1.67 fold) than control. The half-lives ($t_{1/2}$) of pranoprofen in coadministration were increased significantly (p<0.05) by 20-mg/kg aspirin. Based on these results, we might be considered that the pharmacokinetics of pranoprofen would be affected by coadministration of aspirin, by inhibit its metabolism in the liver and the tubular secretion of the kidney with the same acidic property. It should take into consideration in dosage regimen of pranoprofen when coadministration of pranoprofen with aspirin in treatment of rheumatoid arthritis.