The neurotoxic b-amyloid peptide is known to have a negative impact on synaptic plasticity in the CNS. However, its exact mode of action is yet unknown. We investigated whether b-amyloid induces changes in neural activity. We monitored sensitization by measuring amplitude of excitatory post-synaptic potentials (EPSP) in command neurons of Helix lucorum that are responsible for avoidance reactions. EPSPs were similar to the control group when 6*10-6M of b-amyloid (25–35) was bath applied for 0.5 hours. However, after 3 hours of b-amyloid (25–35) treatment no increase in EPSP amplitude was observed. Our results clearly demonstrate that b-amyloid suppress the sensitization of synaptic responses in command neurons in the snail CNS. In behavioral experiments three snail groups were used. The first group was control. The snails from both other groups were injected with b-amyloid peptide (25–35) in concentration 10–4M and volume 0.2 ml. Sensitization sessions for the experimental groups was carried out on the first and third days after b-amyloid peptide treatment. The behavioral experiments on the 1st and 3rd days after b-amyloid peptide injection showed a significant decrease of sensitization (P<0.001) in comparison with control group snails. This results show that neurotoxic fragment of b-amyloid peptide (25–35) may play a significant role in mechanisms of synaptic plasticity.