The responses of lumbosacral motoneurones to iterative dorsal root (DR) stimulation were studied in unanesthetized, spinalized kittens and adult cats by means of intracellular microelectrodes. The resting membrane potentials were found to increase in magnitude from birth up to 1.5 months of age. The action potentials increased in amplitude but decreased in duration with increasing age. In young kittens many cells, for several reasons considered to be motoneurones, failed to be invaded by antidromic spikes. In kittens less than 10 days old the majority of the action potentials did not show an overshoot. The amplitude of orthodromic spikes could be varied in kittens by varying the stimulation strength, probably due to summation of dendritic spikes. In young kittens supramaximal DR stimulation produced large depolarizations which generally decreased in magnitude with increasing age. Spikes elicited during such depolarization were often considerably reduced in amplitude (sometimes more than 50%). In young kittens almost all neurones responded with a monosynaptic spike to each DR‐stimulus at a frequency of 2/sec. This property gradually disappeared with increasing age, especially in extensor motoneurones. The findings are discussed in the light of some recent studies on the postnatal changes in morphology and physiology of the cat spinal cord.

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