Impulse trains evoked by stimulation of the hindlimb nerves were studied in group A fibres at the lumbar level of spinal cord in anesthetized cats. Random (Poissonian) trains fo stimuli and trains with modified structure were used. With suprathreshold stimulation the impulse trains in the nerve fibres differed from the stimulus trains only by the absence of the shortest (less than 1.0 divided by 1.5 ms) intervals caused by refracteriness. Substantial differencies between the impulse and stimulus trains occurred when the stimulus intensity was decreased to a near-threshold level and mean frequency of stimulation was higher than 10 per second. Differences were found in interval histograms, intensity functions, correlation coefficients for successive intervals. Afterdischarge processes in the nerve fibres are supposed to be the main reason of changes in the impulse train structure.