Destruction of the hippocampal perforant path fibers reduces the glutamate content of a crude synaptosomal fraction of the rat fascia dentata by about 40%, but does not reduce the aspartate content. This result supports the hypothesis that the perforant path fibers use glutamate as their transmitter. Since a perforant path lesion reduces the glutamate content of dentate homogenates to a much lesser degree, the reduction in synaptosomal glutamate may be offset by an increase in extraterminal glutamate stores. Thus, when evaluating the possible transmitter role of glutamate or aspartate, one can probably gain more definitive information by measuring the glutamate and aspartate content of a synaptosomal preparation than of the target region as a whole.