1. A grease-gap technique has been used to measure d.c. potentials, in response to the application of excitatory amino acids and electrical stimulation of the Schaffer collateral-commissural pathway, in the CA1 region of rat hippocampal slices. The actions of L-glutamate (L-Glu) have been quantified and compared to those of structurally related compounds. 2. Perfusion of L-Glu (90s applications) depolarized the tissue with a threshold of approximately 50 microM and a maximum response in excess of 10 mM. L-Aspartate (L-Asp) produced a similar dose-response relationship. By comparison N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) were more potent excitants, producing dose-dependent depolarizations over the range 2-50 microM. 3. Application of the agonists depressed the amplitude of electrically-evoked synaptic responses; an effect that presumably reflects depolarization of neuronal tissue. However, for a given agonist-induced d.c. potential. L-Glu or L-Asp caused smaller depressions of synaptic responses than did either NMDA or AMPA. 4. The combined application of 50 microM D-2-amino-5-phosphonopentanoate (AP5) and 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) substantially depressed synaptic responses and antagonized responses to NMDA and AMPA producing mean (+/- s.e.) dose-ratios of 12.2 +/- 1.2 and 7.0 +/- 0.8, respectively. However, these compounds produced minimal antagonism of responses to L-Glu and L-Asp (dose-ratios of 1.5 +/- 0.1 and 1.5 +/- 0.2, respectively). 5. Responses to the stereoisomers of homocysteate (HCA) were compared over the range 50 microM to 10 mM. D-HCA was approximately 3.6 times more potent than L-HCA and was antagonized to a greater extent by the combined application of 50 microM AP5 and 10 microM CNQX; the dose ratios were 8.7 + 0.8 and 5.1 + 0.9 for the D- and L- isomers, respectively. 6. The application of high doses of an excitant (e.g., 50mM L-Glu or 5mM D-HCA) caused an irreversible loss of sensitivity to NMDA and AMPA and abolished synaptic transmission. Responses to the other excitants were depressed by this excitotoxic lesion in the following order: D-HCA > L-HCA > L-Glu = LAsp. In slices treated in this manner, L-Glu, L-Asp and L-HCA produced very similar dose-response curves. 7. Some slices were unresponsive to NMDA, AMPA and electrical stimulation from the onset of the experiment but had sensitivity to L-Glu, L-Asp and L-HCA similar to that of slices that had received an excitotoxic lesion. 8. Slices that were experimentally lesioned, such that they became unresponsive to synaptic stimulation, AMPA and NMDA, were depolarized by a variety of compounds when tested at 5 mm. The D- and L- enantiomers of Asp and Glu produced similar responses and were slightly more active than quisqualate and threo-hydroxy-L-aspartate (THLA). Glycine, L-serine, D-serine, L-alanine and 4-aminobutanoate (GABA) elicited responses 15-30% of the size, while L-lysine, alpha-amino-isobutanoate and L-proline produced depolarizations of less than 10% of the size of those induced by 5mM L-Glu. NMDA and dihydrokainate were either inactive or induced small negative shifts in the d.c. potential. 9. We conclude that when hippocampal slices are perfused with L-Glu (or L-Asp) the main source of the depolarization is due to an interaction with an electrogenic carrier system.