Synapsin I, one of the major synaptic proteins, is thought to associate with synaptic vesicles and to play a regulatory role in neurotransmitter release. In mature neurons, it is concentrated almost exclusively in presynaptic nerve endings. Here, we studied the subcellular localization of synapsin I during the development of rat cerebellar cortices by immunocytochemistry, using anti-synapsin I antibodies and found that during the development of rat cerebellar cortices it tentatively exists in the dendritic growth cones of immature internal granule cells and in the axonal growth cones of mossy fibers as well as mature presynaptic endings. Also, we found that synapsin I, in the axonal and dendritic growth cones does not necessarily associate with vesicles, but rather with fuzzy filamentous structures in the cytoplasm. In search of the structure of synapsin I in vivo, we employed the quick-freeze, deep-etch technique after immunogold labeling. Synapsin I seems to thereby connect synaptic vesicles or anchor them to cytoskeletons in presynaptic endings.