Monoclonal antibodies directed against a neuronal cell surface heparan sulfate proteoglycan and against a synaptic vesicle protein were used to study the postnatal development of ganglionic neurons and synapses in the rat superior cervical ganglion. Antigen levels in developing ganglia were quantitated by radioimmune assays. Localization of antigens in adult and developing ganglia was carried out using peroxidase-antiperoxidase immunocytochemistry at the light microscopic level. Ultrastructural staining patterns in adult ganglia also were studied. The time course of antigen increases parallels those in previous reports on the accumulation of neurotransmitter enzymes within the ganglion. Both synaptic and surface antigens increase postnatally, with the most rapid changes occurring during the 2nd week. Antibodies stain adult tissue in patterns consistent with the expected distribution of antigens: antibodies directed against synaptic vesicles stain synaptic terminals and cell cytoplasm and those directed against surface proteoglycan stain the plasma membranes of neuronal cell bodies and processes. Variable staining of the cell cytoplasm also is observed. No apparent changes in antigen distribution are observed with the light microscope during development. Variations in the time course of the development of antigens associated with different portions of the proteoglycan molecule suggest that the intracellular processing of the molecule may vary during development.