A quantitative study of subthreshold excitatory postsynaptic potentials (EPSP) recorded intracellularly in giant premotor interneurons of the terrestrial snail was carried out after the formation of a conditioned defensive reflex of food aversion in snails with increased level of serotonin. The results showed a significant increase in the number of low-amplitude EPSP with an amplitude from 0.3 to 0.5 mV in the giant premotor interneurons of defensive behavior after learning and increasing the level of serotonin. The observed increase in the number of EPSP may indicate either an increase in the number of action potentials in the corresponding presynaptic neurons or an increase in the amplitudes of the EPSP that were previously undetectable.