Abstract Sustained negative potentials were recorded in the ventral horn of the cat spinal cord during current balanced extracellular iontophoresis of excitatory amino acids. The potentials (referred to a distant indifferent electrode) were measured by an extracellular microelectrode. These focal potentials (FPs) were evoked by dl-homocysteate, l-glutamate, N-methyl- d-aspartate and kainate. These FPs are not an artifact of extracellular microiontophoresis. Their time course is correlated with the depolarization of spinal motoneurones by excitatory amino acids. During iontophoresis of kainate, FPs can be as large as 50 mV and can be recorded for up to 1 mm from the site of drug application. The FP and depolarization caused by kainate were usually irreversible. The depolarization of motoneurones evoked by excitatory amino acids is very much larger when recorded as a ‘transmembrane potential’ (i.e. the potential of an intracellular electrode minus the potential of a local extracellular electrode) rather than as a ‘classical’ intracellular potential (i.e. referred to a distant reference electrode). Possible mechanisms for the generation of the FP are discussed. It is suggested that FP may be recorded routinely during microiontophoretic studies employing extracellular recording of neuronal activity. The application of the FP as a measure of cell depolarization during pharmacological studies of excitatory amino acids and agents that block their action is discussed.