Neurological mutations affecting the cerebellum of the mouse have offered the possibility to study some of the cellular mechanisms involved in the establishment of synaptic connections (see in Sotelo, 1990). Indeed, these mutations provoke through well-defined lesions, the disruption of the normal processes of synapse formation and, by examination of the perturbations in the adult cerebellar connectivity, it is possible to unravel some of the numerous and intricate cellular interactions taking place during synaptogenesis. Furthermore, some of these mutants primarily affect Purkinje cells, the pivotal elements of the cerebellar cortex and its only output, inducing their degeneration. These Purkinje cell-deficient cerebella offer an optimal material to try--by grafting experiments--to replace the missing neurons, and to analyze synaptogenic processes between neuronal partners of different biological ages: the host adult neurons and the embryonary grafted Purkinje cells (Sotelo et al., 1990). The aim of this paper is to summarize some of the work carried out in my laboratory concerning the two above-mentioned topics.