Previous studies have revealed that polyribosomes are selectively localized beneath post-synaptic sites on central nervous system (CNS) neurons, and are particularly prominent during periods of synapse growth. The present study evaluates whether polyribosomes are most prominent at a consistent time in the developmental history of the synapse, or instead at a consistent time in the life of the organism regardless of the state of synaptic maturation (suggesting a globally acting factor). We compare the time course of synaptogenesis and the association between polyribosomes and developing synapses in three regions that develop at different rates: the external and internal blades of the dentate gyrus, and the CA1 region of the hippocampus proper. Each region was examined electron microscopically at 1, 4, 7, 10, 15, 20, 28 and over 120 days of age, evaluating: (1) synapse density (the number of synaptic profiles/area of neuropil), (2) the width of the neuropil layers, (3) the proportion of synapses with underlying polyribosomes, and (4) the number of polyribosome-containing synapses/area of neuropil. As anticipated on the basis of the differences in cytogenesis, the time course of synaptogenesis was different in the three regions. In the external blade of the dentate gyrus, synapse density increased in a nearly linear fashion between birth and 15 days of age, and then continued to increase at a somewhat slower rate until 28 days of age. Synapse development in the internal blade was delayed by several days in comparison to the external blade. In CA1, synapse density increased slowly between 1 and 7 days, and then at a rapid rate between 7 and 28 days of age. In all three regions, the proportion of synapses with underlying polyribosomes was highest between 1 and 7 days of age, and then decreased as synapse density increased. However, the peak in the number of polyribosome-containing synapses/unit area of neuropil occurred at different times in the three regions (4-7 days of age in the external blade of the dentate gyrus and in CA1, and 20 days of age in the internal blade). In addition to further defining the relationship between polyribosomes and developing synapses, the present study provides a data base on the time course of synapse development in the hippocampus and dentate gyrus, which will be useful for comparisons with other measures.