In a previous study, we reported a retardation in process outgrowth and synapse formation in cultured hippocampal neurons from synapsin I-deficient mice. Here we investigated whether this delay in synaptogenesis was attributable to pre- or postsynaptic elements. The experimental paradigm used in this study involved the establishment of heterochronic cocultures of neurons from wild-type and synapsin I-deficient neurons established synapses with mature wild-type postsynaptic elements after 24 and 72 h, respectively. In contrast, synapsin I-deficient postsynaptic elements were able to receive synapses only after 9 days in culture, representing a 5-day delay compared to controls. The results suggest a broad role of synapsin I in the structural development of the synapse, participating directly or indirectly in the maturation of both presynaptic and postsynaptic sites.