Quantitative electron microscopy was used to examine the ability of aged (2-year-old) and young adult (90-day-old) rats to replace those synapses lost (85-90%) in the outer two-thirds of the molecular layer of the dentate gyrus after a complete unilateral lesion of the entorhinal cortex. In aged rats the synaptic density is significantly lower than that of young adults at 10 days postlesion. Synaptic replacement begins between 2 and 4 days postlesion in young adults, whereas there is a delay until after 10 days postlesion in aged rats. Once synapse replacement begins in aged rats, the rate of synapse reappearance is about equal that of young adults. Thus the initial 10 days postlesion appears critical to growth of responding afferents and reformation of synaptic contacts. Analysis of synapses in terms of noncomplex and complex synaptic types shows that the noncomplex type accounts for the significant synaptic density difference between the two age groups. Replacement of complex synapses is nearly indistinguishable between age groups and is complete by 60 days postlesion. In contrast the initial replacement rate of noncomplex synapses in aged rats is much slower than young adults, though the control synaptic density is achieved by the end of the time course.