Metformin, a widely prescribed anti-hyperglycemic agent, is very hydrophilic with net positive charge at physiological pH, and thus should be poorly absorbed. Instead, the drug is well absorbed (oral bioavailability of 40-60%), although the absorption is dose-dependent and variable; the drug accumulates in enterocytes during oral absorption. To date, the transport processes associated with the intestinal absorption of metformin are poorly understood. This dissertation work describes an unusual and novel intestinal absorption mechanism for metformin. The absorption mechanism involves two-way transport of metformin across the apical membrane of enterocytes that is mediated by cation-selective transporters, and facilitated diffusion across the paracellular route, working in concert to yield high and sustained absorption. Metformin absorption was evaluated in the well established model for intestinal epithelium, Caco-2 cell monolayers. Metformin was efficiently transported across the apical membrane by bidirectional cation-selective transporters; however, the drug accumulated in the cells due to inefficient egress across the basolateral membrane. Consequently, the absorptive transport was almost exclusively through the paracellular route; however, the paracellular transport contained a distinct saturable component. Evidence is presented to show that the mechanism responsible for the observed saturable paracellular transport involves electrostatic interactions between positively charged metformin and negatively charged amino acid residues on the pore-forming tight-junction protein, claudin-2. Treating Caco-2 cells with the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3, selectively induced claudin-2 in preference to other tight junction proteins, and concurrently increased paracellular transport of metformin. Overexpression of claudin-2 in renal epithelial cells, LLC-PK1, caused size-dependent increase in paracellular transport of small organic cations, further supporting the role of claudin-2 in facilitating paracellular transport of hydrophilic cationic compounds. By employing a novel chemical inhibition scheme, it was revealed that both the organic cation transporter 1 (hOCT1) and the plasma membrane monoamine transporter (PMAT) were involved in the apical uptake/efflux of metformin in Caco-2 cell monolayers. Taken together, these results suggest a novel mechanism to explain how a hydrophilic cation like metformin is absorbed efficiently, though it uses the inefficient paracellular route for absorption. It is hypothesized that metformin is taken up into enterocytes via apical cation-selective transporters, hOCT1 and PMAT, and accumulates in the cells because of inefficient basolateral egress due to the lack of cation-selective efflux transporters. At each segment of the intestine, a small fraction of the metformin dose is absorbed via the paracellular route, facilitated by claudin-2, while a significant portion of the dose is taken up into the cells. Drug is then effluxed back into the lumen as the dose of the drug travels forward, taken up into distal enterocytes, or absorbed through the paracellular space. The apical transporters function to sequester the drug and allow for multiple opportunities to be absorbed by the paracellular route; thus, increasing the residence time in the intestine enabling efficient absorption. This dissertation work provides novel insights into the mechanisms associated with intestinal absorption and accumulation of metformin. The absorption mechanism proposed in this dissertation can account for the sustained high exposure of metformin achieved in the primary pharmacological organ, the liver, via the portal circulation. Additionally, the mechanisms proposed here can account for the possible role of the intestine in the pharmacology, gastrointestinal side effects, and adverse events of metformin.