Hypoxia-inducible factor 1 (HIF-1) is composed of HIF-1 alpha and arylhydocarbon nuclear receptor translocator (ARNT), which belong to the basic-helix-loop-helix-Per/ARNT/Sim bHLH-PAS) family of transcription factors. HLF plays key roles in oxygen homeostasis and embryonic cardiovascular development. In this study, we have cloned cDNAs encoding the chick HIF-1 alpha from cultured embryonic ventricular myocytes (CEVM) and then examined its expression in various embryonic tissues. The deduced amino acid sequence of the chick HIF-1 alpha cDNA showed 79% identity with that of the human HIF-1 alpha cDNA. In contrast, sequence homology between the chick HIF-1 alpha and endothelial PAS protein 1 (EPAS1), another member of the bHLH-PAS proteins, was only low (49%). HIF-1 alpha mRNA was expressed abundantly in CEVM., but scarcely in the liver, which was quite different from expression pattern of EPAS1 mRNA. These data suggest that HIF-1 alpha may be involved in embryonic cardiovascular development. HIF-1 alpha and EPAS1 may play distinct roles during developmental stages.