Synapsin 1 is one of a family of phosphoproteins located on small synaptic vesicles (SSV) in the presynaptic terminal, and probably plays a critical role in the process of neuronal exocytosis by providing regulated linkages between SSV and the cytoskeleton. Two forms of synapsin 1 are produced from a single gene by differential mRNA splicing: 1a, 706 amino acid residues, and 1b, 670 residues. Synapsin 1 has two structural domains, a globular N-terminal head domain and an elongated tail domain. Electron microscopy of nerve terminals in situ and reconstitution studies in vitro indicates that synapsin 1 can interact with microtubules, microfilaments and brain spectrin. In vitro, synapsin 1 can bundle microtubules. This could either occur by synapsin 1 being at least bivalent for microtubules, or by univalent synapsin 1 molecules aggregating to form complexes that are more than univalent. To resolve this question, we have taken the approach of preparing defined fragments of synapsin 1 from each structural domain and analysing them for tubulin-binding activity. Our results show that there are tubulin-binding sites in both head and tail domains. We conclude that synapsin 1 monomers should be able to cross-link microtubules.