The effects of the separate and combined application of hypoxia and antisense oligonucleotides (ASO) against hypoxia inducible factor 1alpha (HIF1A) on cancer cells were examined. Experiments were carried out on human ovarian carcinoma cells in four series: (1) control [Normoxia (5% CO2 in air), no treatment], (2) hypoxia (1% O2, 5% CO2, and 94% N2 for 48 h), (3) treatment with ASO targeted to HIF1A (48 h), and (4) combined action of hypoxia and ASO. After treatment, the following processes and factors were monitored: apoptosis, cellular metabolism and viability, expression of genes encoding HIF1A, von Hippel-Lindau tumor suppressor protein (VHL), and genes responsible for cell death induction and antiapoptotic defense (P53, BCL2, BAX, and caspases 9 and 3). Expression of caspase 9 and HIF1A protein was confirmed by Western blotting. Liposomes were used as a delivery system of HIF1A ASO. It was found that hypoxia alone significantly disturbed cellular metabolism, reducing the level of respiration by 50% when compared with control. Hypoxia induced apoptosis by upregulating the P53-, BAX-, and caspase-dependent cell death pathways, while activating cellular antiapoptotic defense by the overexpression of BCL2 protein. Both opposing effects were dependent on the overexpression of hypoxia inducible factor. We conclude that hypoxia induces a bimodal effect, simultaneously promoting cell death and activating cellular resistance. The downregulation of HIF1A promoted cell death induction and prevented activation of cellular defense by hypoxia. This suggests that HIF1A is a potential candidate for anticancer therapeutic targeting.