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Hypothalamo-hypophyseal neurosecretory system (HHNS) of 1-30 days old lemmings Dicrostonyx torquatus from the Wrangel Island in the year of a decreased but still highly dense population was studied by paraldehyde fucshin method and immunohistochemically using antisera to vasopressin and oxytocin. In the period of first two weeks of postnatal development the rate of HHNS formation was found to have significant individual differences and to be slower than HHNS formation in laboratory rats due to less stable and suitable environmental conditions. Like in rats the rate of differentiation of the supraoptic nucleus in lemmings was faster than that of the paraventricular one. Formation of the median eminence was retarded as compared to the hypophyseal posterior lobe. Vasopressin-immunoactive structures appeared earlier than oxytocin-immunoreactive ones both in the neurosecretory centers and in the neurohaemal regions. By 21-30 days the development the HHNS system was practically completed and the levels of maturation of studied structures in different individuals became closer. In this period an accumulation of neurosecretory material was observed in all parts of the HHNS which is probably connected with activation of the system under the condition of stress due to a high population density.