INFOBOX The GI sampling technique is an invasive but relatively straightforward way to study the intraluminal drug concentrations in the human GI tract. Depending on the type of question, single or multiple catheters can be placed in the stomach and/or small intestine to sample fluids from these regions and analyze drug concentrations. In most cases, the GI sampling technique is used to assess the influence of the drug and formulation properties on luminal drug concentrations. If combined with additional methods or techniques (e.g., blood sampling, manometry, MRI), this information can also be linked to aspects such as systemic drug exposure or GI motility. Furthermore, GI sampling data are often used as reference data for physiologically relevant in vitro and in silico models.