The effects of an intensive 7 day glucocorticoid (e.g. methylprednisolone) regimen have been studied on the electrical properties of cat lumbar spinal motor neurons via intracellular recording. The results reported in this paper have shown that glucocorticoid dosing produces numerous effects on motor neuron excitability and impulse generation and conduction. These include a resting hyperpolarization, a slowed conduction of an antidromic action potential through the initial axon segment and an increased threshold, a slowed rate of depolarization and a prolonged refractoriness of the soma-dendritic portion of the neuron. On the other hand, the excitability of the initial axon segment, where the nerve impulse is physiologically triggered, is increased as demonstrated by a decrease in the rheobasic current and an increased slope of the current-frequency relationship for repetitive discharge. An additional effect of the steroid is to augment the action potential after-depolarization. These results suggest a complex glucocorticoid action on specific ionic mechanisms which is discussed along with possible neurological and psychiatric implications.