Summary (1) A giant dopamine-containing cell (GDC) in the central nervous system of Planorbis corneus produces excitatory or inhibitory postsynaptic potentials in a number of different follower neurones; the potentials appear to be produced by direct connexions from the GDC and to result from release of dopamine. The effects of bathing the isolated central nervous system in 6-hydroxydopamine (6-OHDA) were observed on the GDC and on transmission from it. (2) 6-OHDA produced a postsynaptic blockade of transmission from the GDC by blocking the dopamine receptors of the follower neurones. The action was specific and more effective for inhibitory transmission than excitatory; it was reversible up to about 1 h exposure to 6-OHDA. (3) 6-OHDA caused an increase in catecholamine-specific fluorescence in the GDC perikaryon. No concomitant changes in ultrastructure were observed, although certain axon processes in areas of neuropile containing presynaptic endings of the GDC showed a marked increase in vesicle granulation, and subsequent degeneration.