It was previously shown that the contralateral (relative to preferred side of turns) optokinetic stimulation and ipsilateral eye enucleation cause a significant, 2- to 4-fold reduction of the ventral dendrite (VD) volume in one of two goldfish Mauthner neurons (MN) that becomes more active functionally. In this study, we investigated the MN ultrastructure after mentioned unilateral visual effects. In both cases, devastation of the afferent synapses was detected along the full length of the reduced VD, with simultaneous compaction of its cytoskeleton, in contrast to those of VD of the contralateral MN and of lateral dendrites and cell bodies of both neurons. It is suggested that the depleted synapses belong to the excitatory visual afferent input, and both cytoskeletal and synaptic mechanisms are involved in the regulation of MN functional activity through VD.