The fast, tightly regulated release of neurotransmitters from presynaptic nerve terminals is effected by a complex molecular apparatus. The precise roles of the various proteins involved remain largely conjectural. Cysteine string proteins (CSPs) are novel synaptic vesicle components that have been conserved in evolution. They are characterized by an N-terminus 'J'-domain and a central, multiply palmitoylated string of cysteine residues. Vertebrate CSPs have been implicated in a functional interaction of synaptic vesicles with presynaptic Ca2+ channels. Genetic 'knockout' of CSPs in Drosophila results in a temperature-sensitive breakdown of elicited transmitter release. Here we try to integrate these observations into speculative functional models on the role of this new protein family in synaptic vesicle exocytosis.