Serial thin sections of the mouse olfactory bulb from the fourteenth day of gestation (E14) to postnatal to 44 (P44) have been examined in order to study morphogenesis of individual synaptic junctions. At the initiation of synapse formation structures are found that resemble postsynaptic densities but are facing extracellular space or unmodified processes. Transition forms between these isolated postsynaptic densities and undoubted synapses have been found. These observations as well as quantitative studies support the hypothesis that isolated postsynaptic densities can form independently and be converted to synapses when a presynaptic specialization develops opposite them. Detailed studies of olfactory axodendritic synaptogenesis throughout the entire developmental period suggests strongly that these asymmetrical synapses pass through an immature symmetrical phase: (1) symmetrical olfactory axodendritic synapses are found in significantly higher concentration on axonal and dendritic growth cones than on more common processes; (2) the number of symmetrical synapses is correlated with the rate of formation of new synapses determined previously. The time for a recognizable symmetrical synapse to be transformed into a recognizable asymmetrical one has been calculated to be 9--10 hours. A scheme of synapse formation in the CNS has been proposed in which a post-synaptic structure forms independently followed by aggregation of pre-existing presynaptic components into a presynaptic specialization. Different morphogenetic sequences of synapse formation from region to region are attributed simply to different relative rates in the development of the postsynaptic density and the presynaptic specialization.