Electrophysiological and neurochemical studies have been made in identifiable neurones in the central nervous systems of the gastropod molluscs Helix pomatia, Helix aspersa and Planorbis corneus. Using a technique of intracellular injection of radioactively labelled precursors, the neurotransmitter synthesising capabilities of several neurones have been investigated. Methods have been developed to ensure that radioactive metabolites detected within the nervous system following intracellular injection are within, and confined to, the soma axons, dendrites and terminals of the injected neurones. The results demonstrate that, with one exception some identifiable neurones possess the ability to synthesise either 3H-5-hydroxytryptamine from 3H-tryptophan and 3H-5-hydroxytryptophan or 3H-acetylcholine from 3H-choline but not both transmitters from their precursors. The exception to this observation is the giant serotonin-containing neurones of Helix pomatia and Helix aspersa which can synthesise both 3H-5-hydroxytryptamine and 3H-acetylcholine from their precursors. Moreover this study has shown that both newly synthesised transmitters in the Helix serotonin-containing neurones are transported to the synaptic terminals of the neurone and are probably released there. Thus this neurone appears to be at variance with the concept that a neurone can utilise only one transmitter substance (Dale's Principle). The final part of the thesis is a study of an identifiable -5-hydroxytryptamine-containing neurone in the right pedal ganglion of Planorbis corneus. This neurone has been extensively characterised by neuroanatomical, pharmacological and electrophysiological methods with a view to determining whether it makes monosynaptic connections with other neurones in the central nervous system. Although no direct connections were observed the neurone did modulate spontaneous activity in other neurones. The role of 5-hydroxytryptamine as a neuromodulator is discussed.