The effects of aging on the responsiveness of hippocampal neurons to iontophoretic application of L-glutamate and AMPA were studied in vitro. There were no effects of age on neuronal responses to L-glutamate; however, CA1 pyramidal cells of old rats, but not granule cells in the fascia dentata, showed both a smaller reduction in extracellularly-recorded synaptic responses following application of AMPA (presumably mediated by depolarization), and smaller extracellular "DC" fields (measured by subtracting the DC potentials at the dendrite and soma following AMPA application in the dendrites). To examine the cellular bases of this age-related alteration in AMPA sensitivity, two additional electrophysiological approaches were used: (1) measurement of the amplitude ratios of extracellular EPSP and fiber potential components of the Schaffer collateral-CA1 response; (2) measurement of intracellularly recorded unitary EPSPs and quantal analysis of their fluctuations. The interpretations that would be placed on four hypothetical possible outcomes of such experiments are outlined and assessed in relation to the experimental data. The pattern of results obtained in the present experiments supports the following conclusions: In old rats, individual Schaffer collateral synapses do not appear to have altered AMPA receptor properties, as neither the mean size of the unitary synaptic response nor the apparent quantal size differs between age groups; however, the data do support the conclusion that there are fewer synapses per Schaffer collateral branch in old versus young CA1 pyramidal cells.