SUMMARY1. Intracellular potentials were recorded from inexcitable cells in the superficial dorsal horn of the rat lumbar spinal cord in vitro.2. These cells had a large resting membrane potential (up to -90 mV) but when depolarized were unable to produce action potentials. The resting potential was highly dependent upon extracellular K+ concentrations ([K+]0), with a slope of about 45 mV for a 10-fold change in [K+].3. Electrical stimulation of dorsal roots evoked a slow depolarizing potential lasting for many seconds in the inexcitable cells. The slow potentials were not accompanied by any appreciable change in input conductance, nor were their amplitudes dependent upon the membrane potential.4. The morphology of the inexcitable cells marked by intracellular injection of a fluorescent dye, Lucifer Yellow, was consistent with their being glial cells.5. [K+]. values were measured by K+-sensitive electrodes simultaneously with intracellular glial potentials. Changes in the K+-electrode potential occurred in parallel with slow depolarizing potentials following stimulation of dorsal roots. When the stimulus intensity was altered, changes in the magnitude of the K+-electrode response were correlated with those of the slow potentials.6. Replacement of extracellular Ca2+ by Mg2+ abolished both the slow potentials and K+-electrode responses evoked by afferent stimulation, suggesting that impulses in primary afferent fibres do not directly contribute to the glial slow potentials.7. It is concluded that the slow potentials in glial cells result from a transient increase in [K+]. at the superficial dorsal horn, which is induced by activation of neighbouring internuncial neurones.

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