Today, clusters are often interconnected by long distance networks to compose grids and to provide users with a huge number of available ressources. To write parallel applica- tions, developers are generally using the standard communication library MPI, which has been optimized for clusters. However, two main features of grids - long distance networks and technological heterogeneity - raise the question of MPI efficiency in grids. This paper presents an evaluation and tuning of four recent MPI implementations (MPICH2, MPICH-Madeleine, OpenMPI and YAMPII) in a research grid: Grid'5000. The comparison is based on the execution of pingpong and NAS Parallel Bench- marks. We show that these implementations present several performance differences. We show that YAMPII performs better results than the others. But we argue that executing MPI appli- cations on a grid can be beneficial if some specific parameters are well tuned. The paper details, for each implementation, the tuning leading the best performances.