P-glycoprotein, an ATP-dependent efflux pump, is a membrane transporter that influences the absorption and excretion of drugs. There is a striking overlap between the substrates for CYP3A4 and P-glycoprotein. This study was designed to assess whether udenafil, a substrate of CYP3A4, is also a P-glycoprotein substrate. Udenafil stimulated P-glycoprotein ATPase activity, a putative measure of P-glycoprotein affinity, although with lower affinity than a proven substrate, verapamil. Bidirectional transport studies of udenafil using Caco-2 cell monolayers showed that its efflux (15.9-22.8 ${ imes}$ $10^{-6}$ cm/s) was significantly higher than its influx (3.7-9.1 ${ imes}$ $10^{-6}$ cm/s). P-glycoprotein inhibitors such as cyclosporine, tariquidar and verapamil significantly increased the influx of udenafil and decreased the efflux of udenafil. These results indicate that udenafil is a substrate for P-glycoprotein. The low bioavailability, variable absorption and drug-drug interactions of udenafil may be related to the variability of CYP3A4 and P-glycoprotein expression and to possible CYP3A4 and P-glycoprotein interactions.