The reaction of the hypothalamo-hypophysial neurosecretory system (HHNS) of lemmings to cold (+4 degrees C) was studied by the method of electron microscopy. The animals were kept in cages with nest, but with no wheel to allow running; in cages with a wheel, but with no nest; in cages with no nest and no wheel. No changes in the HHNS reaction were revealed under cold stimulation, if the animals were not deprived of motor activity (running in a wheel), or could hide in a nest. In case of no nest and restricted motor activity exposition to low temperature led to a progressive activation of neurosecretory cells of the supraoptic and paraventricular nuclei, and finally to their exhaustion and degeneration. The intensity of the HHNS reaction to cold is shown to depend on the initial state of the system which varies according to the stages of population cycle. The present study gives grounds to put forward the hypothesis that the adaptation of the Dicrostonyx torquatus Pallas to cold is made possible due to maximal utilization of ecological conditions and behavioral reactions.