Studies in this dissertation were conducted to explore the roles of organic cation transporters (OCTs) in the disposition of N-butylpyridinium Chloride (NBuPy-Cl) and structurally related ILs. Following a single i.v. dose to rats, the blood concentration of NBuPy-Cl and 1-butyl-1-methylpyrrolidinium chloride (BmPy-Cl) decreased in a biphasic manner with a clearance of 3.3 and 7 ml/min, respectively. More than 84% of dosed compounds were excreted in the urine. Depending on the vehicle, the dermal absorption of BmPy-Cl and NBuPy-Cl (5 mg/kg, 125 μg/cm²) was 10-35% at 96 h. Following a single oral (50 mg/kg) administration to rats, the maximum blood concentrations of both ILs were reached in less than 90 min in rats. Most of the orally dosed NBuPy-Cl (62-68 %) was excreted in the urine in 72 h. However, more of the dosed BmPy-Cl was eliminated in the feces Its oral bioavailability was only 47%. The elimination differences between BmPy-Cl and NBuPy-Cl were not altered by the size (0.5, 5, or 50 mg/kg) or frequency (1 or 5 administrations) of oral doses. In all urine and blood samples, only parent compounds were detected. Co-administration of NBuPy-Cl and inulin intravenously to rats revealed that the clearance of NBuPy-Cl exceeded the rat glomerular filtration rate, suggesting a renal secretion processing. The in vitro transport studies demonstrated that NBuPy-Cl, BmPy-Cl and 1-butyl-3-methylimidazolium chloride are substrates (Kt, 9~277 μM), as well as inhibitors (IC₅₀: 0.2~7.5 μM), of rOCT1/2 and hOCT2. Their inhibitory effects increased dramatically with increasing the alkyl chain length. The IC₅₀ values were 0.1, 3.8, 14 and 671 μM (hexyl-, butyl-, ethyl-pyridinium and pyridinium chloride) for rOCT2 mediated metformin transport. Similar structurally related inhibitory kinetics were observed for rOCT1 and hOCT2. In vivo co-administration of NBuPy-Cl prolonged the plasma half-life and reduced renal clearance of the diabetic drug, metformin. In summary, BmPy-Cl and NBuPy-Cl are partially absorbed from gastrointestinal tract. The present in blood is eliminated rapidly in the urine as parent, by renal filtration and OCT-mediated secretion. ILs also compete with other substrates of OCTs and have the potential to alter their pharmacokinetic profiles.