The Lez aquifer (Montpellier, France) developed in a Mediterranean setting, through several stages. Problem : The Messinian crisis is considered as mainly having driven the architecture of the karst conduits. Questions : Is this stage the most important in terms of impact on Lez aquifer architecture, and how does it compare with the previous stages? Tools : To answer to these questions, a numerical model (GODIAG, TOTAL S.A.) has been constructed for the Lez aquifer. It allows to test several scenarii of the evolution of the karstic network through time.