With electron microscope two types of synaptic buttons (SB) are found on the nuerosecretory cells (NSC) in the nucleus preopticus of the mature carp. SB type I contain dense core vesicles and synaptic vesicles, whereas SB type II have only the latter. SB type II is considered to be functionally more active. The density of SB on the NSC body surface varied inversely proportionally to the NSC size, being in average 1.7+/-0.2 on the section of one small NSC in the ventral part of the nucleus preopticus, and 1.0+/-0.1 on the section of a large one in the dorsal part (P less than 0.05). In large NSC type II SB are found more frequent than in small NSC, the synaptic apparatus of NSC in the dorsal part of the nucleus preopticus is in a more active stage, than of NSP in the ventral part. The structural organization of the complex SB-NSC of large cells is more complicated compared to that of small cells. The large cells are supposed to be more differentiated and functionally more active. The synapses of NSC of the nucleus preopticus of the carp are supposed to belong usually to axons of monaminergic neurons.