Cultivation of mesenchymal stem cells (MSC) under physiological oxygen tensions has beneficial effect on their properties. Aim. To examine the effects of mild hypoxic conditions on the morphologic behavior of MSC from human Wharton jelly (WJ-MSC). Methods. WJ-MSC were obtained by an explant method and cultured in gas mixtures containing 3 % O₂, 4 % CO₂ and 93 % nitrogen or argon. At each passage the cultures were fixed and stained with hematoxylin and eosin. The morphometric analysis of microphotographs was performed, and the nuclear-cytoplasmic ratio (NCR) and the width/length ratio were calculated. Results. The difference in NCR and the width/length ratio was detected between WJ-MSC cultures at various passages and between the cultures at the same passage, maintained in different gas mixtures. The highest level of morphological homogeneity was observed at passage 2. At passage 3, the cells with senescent morphology can be observed. The degree of heterogeneity in populations cultured at 3 % O₂ appeared to be lower than under standard conditions. WJ-MSC, cultured in the nitrogen-based gas mixture were the most homogeneous. Conclusions. The changes in WJ-MSC culture morphology were observed at early passages. The level of morphological heterogeneity varied with the cultivation time. The conditions of mild hypoxia had a positive impact on the cultured cells.