Hypoxia is a fundamental part of physical exposure which athletes undergo continually in practice and competition. Hypoxia inducible factor-1 (HIF1a) is known as an important transcription factor in the context of cellular response to hypoxia. As there are still gaps of knowledge in the regulation mechanism of HIF1a, we aimed to investigate the expression of different HIF1a mRNA-variants under defined conditions in vitro and in vivo.In silico analysis of HIF1a-gene yielded an polygenetically conserved Expressed Sequence Tag (EST) downstream from the 3'end of HIF1a reference mRNA comprising a highly conserved poly-A-signal that suggests the existence of a so far unknown mRNA containing an extended 3'UTR (HIF1a-lv). We succeeded to detect HIF1a-lv in diverse human cell lines. HIF1a-lv appeared to be ubiquitary expressed in different tissues and it is differentially expressed in vitro and in vivo. In vitro trials showed that HIF1a-lv increases significantly 60 min. after induced hypoxia and remains elevated until 20 h, while HIF1a does not change. After endurance exercise HIF1a-lv increased significantly 3.3-fold in vivo while HIF1a stayed at the same level. Neither HIF1a-lv nor HIF1a expression altered by dint of middlelong-term normobar hypoxia (23 h hypoxia) in vivo. Our results suggest that HIF1a-lv is a prolonged variant of the sense mRNA of the HIF1a-gene that is ubiquitary expressed in several tissues and that HIF1a-lv has got regulatory function. We were able to receive new knowledge of transcriptional regulation of HIF1a, but further research should investigate the structure, functions and impacts of HIF1a-lv and putative other mRNA-variants of HIF1a.