The data presented concern synaptic plasticity arising during neuronal response to a single sensory stimulus. The plasticity was studied in identifiable synapses between sensory and command neurones of Helix lucorum using simultaneous intracellular recording. A brief mechanosensory stimulation evoked a train of spikes in an identifiable visceral sensory neurone. Interspike intervals within the train changed from short (tens ms) to long (s). Each spike elicited an elementary EPSP in an identifiable command neurone. The amplitude of elementary EPSPs varied during a train of action potentials. Initial high frequency depression was followed by posttetanic potentiation and subsequent low frequency depression. The result of integration of the three plastic processes has been named immediate plasticity. It has been shown that a compound EPSP elicited in a postsynaptic command neurone by a single sensory stimulus depends on the immediate plastic changes of elementary EPSPs.