1. A giant dopamine-containing cell, situated in the left pedal ganglion of the water snail Planorbis corneus, was identified in isolated living preparations of the central nervous system. Spectrophotofluorimetric analysis confirms that the cell contains dopamine, whereas noradrenaline appears to be absent. The cell is unique in being a repeatedly identifiable dopamine-containing neurone. 2. Stimulation of the giant dopamine-containing cell resulted in excitatory, inhibitory or biphasic (depolarizing-hyperpolarizing) synaptic potentials in a number of follower neurones. The duration of the e.p.s.p.s and i.p.s.p.s was 0-3-5 sec; they ranged from barely detectable responses to ones 7 mV in amplitude in different cells. The depolarizing phase of a biphasic synaptic potential (b.p.s.p.) was usually less than 1 mV in amplitude (max. 3mV) and lasted 40-400 msec. The latency of i.p.s.p.s was long (70-120 msec) compared with that of e.p.s.p.s and b.p.s.p.s (20 msec). Abolition of the depolarizing phase of b.p.s.ps. by tubocurarine left a long-latency (70-120 msec) i.p.s.p. All responses showed summation and marked facilitation. 3. Evidence is presented that the post-synaptic potentials are produced by direct connections from the giant cell and result from a release of dopamine. Of eight putative transmitter substances tested on these different groups of neurones, only dopamine produced a potential change which in each case was of the same polarity as the post-synaptic potential when this was monophasic. However, generally applied dopamine produced only a hyperpolarization in follower cells showing b.p.s.p.s. This result is probably partly due to rapid desensitization of the receptors mediating the depolarization and also to a masking of the depolarization by the more effective hyperpolarizing response. 4. Erogometrine and 6-hydroxydopamine specifically antagonized the i.p.s.p.s and dopamine receptors mediating inhibition. Neither the e.p.s.p.s nor the excitatory dopamine response were blocked by high concentrations of hexamethonium. Hexamethonium was also ineffective in blocking the depolarizing phase of a b.p.s.p., which was, however, selectively eliminated by tubocurarine. 5. It is suggested that dopamine is the transmitter released from the giant cell and that it can mediate excitatory, inhibitory or biphasic responses in different follower neurones.