1. The onset and development of transmission has been studied electro-physiologically in the isolated chick ciliary ganglion from Stage 25 (Hamburger & Hamilton, 1951) until 28 days after hatching. Ultrastructure of the synapses was concomitantly investigated.2. Synaptic transmission began at Stage 26(1/2) and was 100% in both cell groups, ciliary and choroid, by Stage 33. It was initially chemical until Stage 41 when effective electrical coupling first appeared in the ciliary population. The proportion of electrically transmitting synapses increased to 80% by 1-2 days post-hatching.3. Few morphological synapses were present at Stage 33(1/2) when all ganglion cells were transmitting. A scarcity of synaptic vesicles persisted until late in embryonic development when all ciliary cells possessed calyces. At hatching the calyces were filled with synaptic vesicles.4. Initial synaptic contacts were by fine terminal branches often on the intricate processes of early ganglion cells. Calyces formed from Stage 36(1/2) and there was a concomitant retraction of ganglion cell processes, so that by Stage 40 all ciliary cells had simple calyces. The calyx was a transitory structure, which from the first week post-hatching began to break up into a cluster of boutons.5. Chemical post-synaptic potentials (PSPs) were at Stage 40 long (30 x the membrane time constant) and further prolonged by eserine. By Stage 43, PSPs had become markedly shortened and were unaffected by eserine. No simple explanation can be offered for the changes in PSP time course and sensitivity to anticholinesterases during development.6. Intracellular records from Stage 40 ciliary cells, which all possess calyces, showed 1-2 mV amplitude, diphasic, fast decaying electrical coupling potentials (CPs). Later in development the CPs became 20-40 mV amplitude, more slowly decaying and monophasic. This seemed to be correlated with faster presynaptic conduction velocities and myelination of the cell soma. Such changes in CPs may reflect a shift from capacitative to more resistive coupling and point to several factors contributing in varying degrees to the electrical transmission.7. Presynaptic fibres innervating ciliary cells were from the start of lower threshold and faster conduction velocity than those innervating ciliary cells, as occurred in the adult. It is concluded that these preganglionic fibres were probably specified by the time transmission starts and that they selectively innervated the proper post-synaptic cells.