Here we investigated the pharmacokinetic (PK) profiles and the role of P-glycoprotein (P-gp) on the intestinal absorption of compound A in rats. In spite of the high solubility, good permeability in a parallel artificial membrane permeability assay (PAMPA) (apparent permeability (Papp) = 14.1×10-6 cm/sec at pH 6.5) and the moderate metabolic rate in liver microsomes, in PK (i.v. and p.o.) studies compound A exhibited high total clearance (CLt) and poor bioavailability (< 10%). However, its metabolites were not detected in plasma after oral administration. Furthermore, relatively high excretion into the urine (27%) and bile (9%) was detected after i.v. administration. FaFg of Compound A was extremely low as well (0.03). This was determined using the portal-systemic concentration difference method, which calculates the difference in AUC between the portal vein and the systemic circulation. We observed a nonlinear increase in compound A’s plasma concentration when administered orally at 3 mg/kg or 10 mg/kg. Furthermore, transport studies in MDCK cells expressing MDR1 revealed that compound A was a P-gp substrate. Compound A’s Papp in Caco-2 cells was only 0.6×10-6 cm/sec in the absorptive direction at pH 6.5. These findings suggested that P-gp accounts for compound A’s poor FaFg. To clarify whether P-gp limits compound A’s intestinal absorption, a PK (i.v. and p.o.) study was performed using Mdr1a-deficient mice. Compound A’s bioavailability in Mdr1a-deficient mice (20%) was 7-fold higher than wild type (3%). We also performed a drug-drug interaction study in rats using elacridar to inhibit P-gp. Concurrently administering elacridar increased compound A’s bioavailability approximately 6-fold, while no change in CLt was observed. These studies clearly demonstrate that P-gp caused, in part, poor bioavailability of compound A in rats. Limited intestinal absorption due to P-gp is likely to affect the pharmacokinetics of other P-gp substrates in rats and humans alike.