Specific synaptic connections are the sine qua non of the integrative nature of nervous system function. During development, many different mechanisms regulate the ultimate synaptic connections that will form. A goal of this study is to elucidate cellular and subcellular mechanisms which regulate the formation of chemical synapses. Mechanisms that give rise to the synaptic connections of the nervous system and to the development of presynaptic terminals are ill-defined. A considerable hurdle in such studies is the accessibility of synaptic terminals for direct experimental examination during synapse formation. Using a unique preparation of experimentally tractable giant synaptic terminals (60mu diameter), this study will directly study the formation of presynaptic terminals. In a mature synaptic terminal, action potentials cause the opening of voltage-dependent calcium channels, which leads to an influx of calcium ions, which in turn acts on calcium-dependent secretory apparatus to trigger the release of neurotransmitter. In this study, the appearance of these crucial elements of synaptic terminals will be studied. Furthermore, by studying presynaptic development in two different identified neurons of Helisoma this project will investigate cellular and subcellular mechanisms that control the specificity of synaptic connections.