Using the morphometric techniques, the ultrastructural changes of the afferent synapses on the ventral dendrite of the Mauthner neurons (MNs) were studied after the adaptation of goldfish to long-term fatiguing sensory (visual) stimulation, characterized by the growth of MN resistance. It was shown that after the adaptation, the length of active zones (AZs) in the synapses located on the MN ventral dendrite was significantly reduced by 23%. At the same time, the length the AZs of the excitatory visual synapses was reduced by 29% in comparison with the control, while the length of desmosome-like contacts (DLCs) bordering AZs was increased by 71%. It was also found that the length of AZs in the inhibitory synapses was decreased by 19% after the adaptation, which is consistent with the important role of inhibitory processes in the sensory pathways during the memory formation. Taking into account the actin nature of the DLCs, the basis of the adaptation to the visual stimulation is suggested to be in the presynaptic mechanism of neurotransmitter secretion regulation by actin.