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When administered micro-electrophoretically into the spinal grey matter of cats anaesthetized with pentobarbitone, acidic amino acids known to be neuronal excitants lower the threshold of electrically stimulated muscle and cutaneous primary afferent fibres and terminations. This depolarizing effect was not observed with fibres stimulated in the white matter. Depolarization by micro-electrophoretic potassium and excitant amino acids appeared not be be associated with an alteration in terminal membrane conductance since there was no change in synaptically evoked primary afferent depolarization. Excitant amino acid depolarization was not blocked by the gamma-aminobutyric acid antagonist bicuculline methochloride, but was reduced by selective excitant amino acid antagonists. The results are discussed in relation to the probable absence of specific excitant amino acid receptors on afferent terminals, the depolarizing effect of the amino acids on myelinated fibres and non-myelinated terminals being more likely a consequence of changes in the extracellular medium associated with the depolarization and firing of neurones.