The three consecutive main excitatory neurone projections in the hippocampal formation (the perforant path, the mossy fibres and the pyramidal cell axons) all show acidic amino acid uptake in their terminals. This is in accordance with other data pointing to the transmitter function of glutamate and/or aspartate in the perforant path and pyramidal cell axons. In the mossy fibres the role of these amino acids has so far not been determined. The localization of glutamate and aspartate uptake in the excitatory axon systems contrasts with the localization of GABA uptake in the axons of the inhibitory basket cells and other short axon neurones.