26 practically healthy youths of 16-18 years old not adapted to the mountains earlier were investigated under the conditions of a plain (Kiev) and at the height of 2100 m and 3000 m above sea-level (Chegem). A stay under the conditions of moderate altitude for one month was found to favour amelioration of the characteristics of the functional lability (CFL) of the nervous system, information processing velocity, attention concentration and distribution, short-term memory volume, general subjective feeling as well as shortening the latent periods of visually-motor reactions. Large adaptive shifts of the CFL and more stable positive effect of mountain adaptation after returning to the plain were found in persons with initially high functional lability level in comparison to those with mean or lower values of CFL. Positive connection between the CFL and the degree of adaptive changes of external respiration was observed under the conditions of moderate altitude. Statistically significant connection of the CFL with the respiratory center sensitivity to hypercapnia was found rather than their correlation with the value of ventilatory response to acute hypoxia during gas mixtures respiration.