In this study we examined the impact of 3-day hypoxia of varying degrees on the viability, proliferative and secretory activity of endothelial cells in human umbilical vein (HUVEC). The gas mixture of the three components (%) was used: 1) 10 O2, 5 CO2, 85 Ar; 2) 5 O2, 5 CO2, 90 Ar and 3) 1 O2, 5 CO2, 94 Ar. The HUVEC, cultivated in CO2-incubator under conditions of atmospheric oxygen (21% O2) were the controls. Comprehensive assessment of the results after has shown that 3-day HUVEC cultivating in the presence of 1% O2 led to pathological activation of endotheliocytes: increased NO synthesis combined with the marked secretion of endothelin-1, IL-6, IL-8 and TNF-alpha, sVCAM-1, sE-cadherin and of sE-selectin, VEGF-A and bFGF, and slow proliferation. When HUVEC were cultivated at 10% O2 and 5% O2, the level of basal secretion of the substances listed above was the least against the background of increased proliferative activity. The results showing the changes in the secretory activity of endothelial cells when cultivated under the conditions of atmospheric oxygen levels have demonstrate HUVEC activation, because the secretion of NO, IL-6, IL-8 and von Willebrand factor after 3 days of cultivation in 21% 02 exceeded that in the case of 10 and 5% O2. Thus, a gaseous medium with reduced oxygen content of up to 5% provides more physiological conditions for HUVEC cultivation. The maximum proliferative activity of HUVEC with minimal basal secretion proved such a composition to be comfortable. Increasing the oxygen content to the atmospheric level leads to the activation of endotheliocytes with signs of endothelial dysfunction, and the critical reduction in oxygen to 1% causes the development of endothelial dysfunction and reduces the proliferative potential.