In experiments on the isolated nervous system, responses of the same command neurones of avoidance behaviour to electrical stimulation of the intestinal nerve were compared in adult snails (Helix pomatia) and in snails 1-20 days old. Dynamics of spike reactions of the command neurones to rhythmic nerve stimulation differed in adult and newborn snails. In the neurones of young snails, no sensitization, common in the adult animals, was observed. Study of excitatory input in the command neurones during rhythmic nerve stimulation suggests independence of habituation of synaptic input amplitude and spike response sensitization, because dynamics of habituation coincided in the young and adult snails while spike response underwent sensitization only in adult animals.