It was found in experiments carried out in common snails that applications of a concentrated solution of quinine (10%) to the animal's head lead to long-term (more than 24 h) facilitation of defense reactions. Facilitation of synaptic components of the responses to test stimulation, an increase in the excitability of the membrane, and its depolarization correspond to the behavioral manifestations of long-term sensitization in the command neurons of defense behavior. The degree of manifestation of the sensitization effects depends on the duration of its development. After a one-day development of sensitization, manifestations of signal-specific sensitization, including site-specific sensitization (more pronounced synaptic facilitation in the responses to test stimulation of the segment of the body which had been subjected to the sensitizing influence, as compared with facilitation of responses to stimulation of other points) and modality-specific sensitization (more pronounced facilitation of responses to test stimulation which coincides in modality with the sensitizing stimulus, by comparison with the degree of manifestation of responses to a test stimulation of a different sensory modality), predominate. After a three-day development, signs of generalized sensitization predominated in the command neurons: marked facilitation of responses in all synaptic inputs of the command neurons, an increase in excitability, and depolarization of the membrane.