1. Isolated hemisected spinal cords of the frog have been used to investigate the way excitant amino acids depolarize primary afferent fibres and terminals.2. GABA and excitant amino acids caused depolarization in dorsal roots. But dorsal roots sectioned at the point of exit from the spinal cord responded only to GABA.3. Prolonged application of kainate or N-methyl-D-aspartate to hemicords caused a depolarization of dorsal roots in association with an increased extra-cellular [K(+)]. The two effects decayed with similar time courses. The depolarization recorded from ventral roots was maintained in the presence of the excitants.4. Field potentials, elicited by electrical stimulation of ventral roots and recorded in the ventral horn of Mg blocked preparations, were abolished by prolonged treatment with kainate. Corresponding dorsal horn field potentials elicited by electrical stimulation of dorsal roots were resistant to the presence of kainate.5. Excitability of motoneurones or afferent terminals was measured from the amplitude of action potentials evoked by submaximal cathodal stimulation of ventral or dorsal horns and recorded in ventral or dorsal roots respectively. Prolonged application of kainate to Mg blocked preparations abolished the excitability of motoneurones within 5 min, but the excitability of primary afferent terminals was increased and maintained for several hours.6. These observations support the hypothesis that primary afferent terminals in the frog do not have receptors for excitatory amino acids and that depolarization of terminals induced by excitatory amino acids is mediated through release of K from other cells within the dorsal horn.