Cognitive flexibility is the ability to modify the behavioral response due to change of contingencies in the environment and it’s part of the executive functions. Several psychiatric disorders present cognitive inflexibility, like depression, schizophrenia and autism; and that’s why this topic has turned to be relevant in the field.The serotoninergic system(5-HT) plays a crucial role in different executive functions, and it’s involved in decision-making, planning and action execution. Alterations in this system are found in the previously mentioned diseases. At a functional level, the receptor 2a of serotonin(5-HT2aR) mediates many of the effects of the 5-HT such as sleep, memory, and it’s a key factor in cognitive processes. However, its role in cognitive flexibility is less well understood. The aim of this project is to analyze the role of 5-HT2aR in cognitive flexibility. We will use a series of behavioral paradigms:T-maze, radial-arm maze and intra-extra dimensional shift task, which involve different levels of difficulty in the flexibility component. To analyze the role of 5-HT2aR we will use a genetically modified mouse model recently backcrossed into the C57B/6 strain. We began comparing it with our previous strain, where we found that 5-HT2aR KO mice required more time to reach criteria during the reversal phase of the T-maze.This research could improve our understanding of cognitive flexibility, how it’s modulated, and support the development of potential therapies.