Drug active transport into milk is a major concern for breastfeeding mothers and healthcare providers. Studies from the literature indicated that breast cancer resistance protein (ABCG2) plays an important role in drug transfer into milk. There has been limited study on stereoselective interactions with ABCG2. A mechanistic analysis of flux across cell monolayer model is a critical first step toward extrapolating in vitro results for predicting in vivo disposition (including distribution into milk), drug disposition or drug-drug interactions. The objectives of this thesis were (1) to establish a “Chemical knockout model” in rat for studying drug accumulation into milk, (2) to investigate the impact of stereoselective interaction between ABCG2/Abcg2 and pantoprazole on drug transport in milk, (3) to understand in vitro monolayer flux model using experimental data and a mechanistic mathematical model. Quantitive PCR, Western blotting and immunohistochemistry results indicated that Abcg2 was up-regulated during lactation and localized on apical side of epithelial cells in mammary gland. In vitro and in vivo experiments confirmed that Abcg2 is responsible for nitrofurantoin active transport in rat milk and GF120918 was established as a chemical knockout model. Abcg2 interacts stereoselectively with pantoprazole isomers. A significant different apical flux between two pantoprazole isomers was observed in Abcg2-MDCKII cell line. The milk to serum (M/S) ratio of (-)-pantoprazole was almost 3 times as that of (+)-pantoprazole in lactating rats. Administration GF120918 decreased M/S of (-)-pantoprazole (p<0.001) but not (+)-pantoprazole (p>0.05). A stably transfected ABCG2/Abcg2 overexpressing MDCKII cell line was successfully created and used to explore the theoretical relationships in a monolayer flux model. Based on the profiles of pantoprazole isomer transport, a simple three compartment model for drug transfer into breast milk incorporating the permeability-surface area products for passive diffusion (PSD), paracellular flux (PSPC) and apically efflux ABCG2 (PSA,E) transfection was developed. The mathematical model was developed to more fully understand the interplay of paracellular, passive diffusion, active transport, and flux kinetic parameters (Km, Vmax, IC50 and Ki). This model provided useful insights into the meaning and limitation of parameters obtained from monolayer flux.