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Protein phosphorylation is involved in the regulation of a wide variety of physiological processes in the nervous sys-tem. Studies in which purified protein kinases or kinase inhibitors have been microinjected into defined cells while a specific response is monitored have demonstrated that protein phosphorylation is both necessary and suffi-cient to mediate responses of excitable cells to extracellu-lar signals. The precise molecular mechanisms involved in neuronal signal transduction processes can be further elucidated by identification and characterization of the substrate proteins for the various protein kinases. The roles of three such substrate proteins in signal trans-duction are described in this article: 1) synapsin I, whose phosphorylation increases neurotransmitter re