Spontaneous electrical activity in the isolated hemisected frog spinal cord increased in the presence of the SH reductant dithiothreitol (DTT) was reversibly suppressed by the oxidant 5-5'-dithio-bis-(2 nitrobenzoic acid) (DTNB) and was irreversibly suppressed by the sulphydryl modifying agents N-ethyl-maleimide (NEM) and monobromotrimethylammonio bimane. Glutathione (GSH), an important natural low molecular weight thiol, reversibly suppressed spontaneous activity. Cords pretreated with glutathione and successively exposed to NEM or bimane maintained their normal electrical activity. This indicates that GSH had interacted with the exposed sulphydryls and prevented their reaction with NEM or bimane. Incubation with bimane resulted in fluorescence-labelled neurones in the dorsal and ventral horns, whereas samples pretreated with NEM or with GSH were not labelled. Neurones appeared again fluorescent in cords preincubated with GSH and sequentially exposed to NEM or bimane. Both electrophysiological and histochemical methods indicate that exposed membrane sulphydryls are involved in the genesis and/or modulation of spontaneous electrical potentials.