The most important criteria for evaluating in vitro performance and in vivo behavior of drugs is dissolution tests. It is necessary to provide conditions that best mimic the in vivo media in dissolution tests. The most important condition is the dissolution media used. The dissolution media recommended in pharmacopoeias and guidelines are generally based on the pH values the physiological media. Even though these media are proper for drugs without solubility and dissolution problems, they do not sufficiently reflect in vivo environments for low soluble drugs. However, lipophilicity, solubility, ionization properties of drugs and food-drug interactions should also be considered while selecting a dissolution medium. For this purpose biorelevant media simulating gastrointestinal conditions were developed. Components, osmolality, pH, and buffer capacity of these media are different according to fasting or fed states. In this study, the physiology of gastrointestinal fluids was investigated, the dissolution medium developed to mimic gastric, intestine and colon fluids in fasted and fed states were evaluated. © 2018 Society of Pharmaceutical Sciences of Ankara (FABAD). All rights reserved.