High affinity uptake sites for 3H-labelled amino acids were studied in synaptosome-containing homogenates processed biochemically or in surface autoradiograms of incubated slices of hippocampus. D-aspartate and L-glutamate had apparently identical distributions. In normal rat hippocampus the highest uptake was in the terminal fields of axons from the pyramidal cells of regio inferior and hilus fasciae dentatae, while there was a moderate uptake in the terminal fields of the medial and lateral perforant paths, slight uptake in the mossy fibre layer and no uptake in the terminal fields of the basket cells. Uptake sites for gamma-aminobutyrate were concentrated in the latter fields, and in the most superficial cortical layers. The present method shows no uptake in cell bodies. The uptake activities were strongly inhibited by recognized blockers of (neuronal) high affinity uptake of glutamate or gamma-aminobutyrate. Autoradiographically, several other amino acids showed negligible uptake. The uptake of D-aspartate was reduced by 80% in stratum oriens and stratum radiatum of regio superior 4-14 days (70% at 3 days) after transection of the afferent pyramidal cell axons from the ipsi-and contralateral regio inferior. The reduction was in the number of uptake sites, not in their affinity. Uptake of gamma-aminobutyrate was not reduced. Lesions affecting regio superior caused a loss of D-aspartate uptake in subiculum at a site known to receive hippocampal afferents. Autoradiographically, the uptake of D-aspartate was strongly reduced in the inner zone (i.e. the target zone), but increased in the middle zone of the dentate molecular layer after lesions of the hilus fasciae dentatae. At 4 days and longer after transection of the entorhinal afferents, there was a conspicuous reduction of D-aspartate and L-glutamate uptake in the target zones of both the medial and lateral contingent of these fibres. In the same animals, the terminal zone of afferents from hilus fasciae dentatae had an increased radioactivity and was slightly wider than normally. Concomitantly, the gamma-aminobutyrate uptake was increased in the target zones of the degenerating perforant path fibres. The results demonstrate that uptake sites for D-aspartate and L-glutamate are highly localized in axon terminals of regio inferior pyramidal cells and in perforant path afferents. The latter category of terminals has a lower density of acidic amino acid uptake sites than the former. Uptake sites for gamma-aminobutyrate are localized in terminals of intrinsic neurones, including the axosomatic terminals of basket cells.(ABSTRACT TRUNCATED AT 400 WORDS)