The preoptic-pituitary neurosecretory system (PPNS) plays a key role in the regulation of reproduction, spawning, osmoregulation and fish growth. This study aimed to find out the development features of the PPNS in pink salmon in connection with gonadal sex differentiation and spawning. A comparative morphometric analysis of all parts of the system was carried out in juveniles aged 3–4 months and adults during the spawning season. The ratios of the areas were calculated for the PPNS different segments on the sagittal sections: the cellular parts of the preoptic nucleus, the cyto- and karyometric characteristics of neurosecretory cells. The complex morphometry of neurosecretory, glial and vascular elements in the neurohypophysis was carried out. It was found that during seaward run and the completion of sex differentiation, the main sections of the PPNS were already fully formed and the ratio of all parts of the preoptic nucleus in juveniles and adults was not significantly different (р< 0,05). In juveniles, compared with adults, the relative area of the neurohypophysis was significantly higher (P < 0.05), and level of relative development of the capillary network, the length of neurohemmal contacts and organ hyperemia were comparable. It was shown a high level of formation and functioning of PPNS in juveniles, as well as its important role during seaward run and early gonadogenesis.