Solving the 3D resistivity problem is fundamental to understand the complex response in most environmental contexts. The finite-elements method is a classical tool to solve the dc equations (Coggon 1971 , Pridmore et al. 1981, Holcombe et al. 1984). The forward modelling is often performed in a limited domain, and requires to specify potential boundary conditions. Since the latter are physically unrealistic, they lead to unacceptable errors, especially when dealing with small domains. This paper presents the improvements which can be obtained by using infinite elements in the outer part of the main studied domain, as they permit to discard the Dirichlet condition to infinity.