Ten per cent of the adult number of synapses are present in the neonatal rat superior cervical sympathetic ganglion on day 1 (day of birth taken as day 0). Synapses are formed rapidly over the first postnatal week, and then more slowly, reaching 80% of adult numbers by the end of the third week. Adult numbers are present at the end of the third month. Prominent axosomatic synapses are present for the first two weeks of life, but disappear later. Of the adult numbers of synapses, 20-40% are already present on days 2 and 4, and transection of the preganglionic chain on these days causes disappearance of all synapses by 2 days after operation. However, by 2 months after operation the numbers of synapses are the same as in unoperated ganglia from rats of the same age. Unoccupied postsynaptic densities were not seen either in normal development or after lesions.