Hibernation is a unique physiological state characterized by profound reversible sleep-like state, depression in body temperature and metabolism. The serotonin 5-hydroxytryptamine(1A) (5-HT(1A)) receptor gene sequence in typical seasonal hibernator, ground squirrel (Spermophilus undulatus), was specified. It was found that the fragment encoding the fifth transmembrane domain showed 93.6% of homology with the analogous fragment of the mouse and rat genes and displayed 88.5% homology with the human 5-HT(1A) receptor gene. Using primers designed on the basis of obtained sequence, the expression of 5-HT(1A) receptor gene in the brain regions in active, entering into hibernation, hibernating and coming out of hibernation ground squirrels was investigated. Significant structure-specific changes were revealed in the 5-HT(1A) messenger RNA (mRNA) level in entry into hibernation and in arousal. An increase in the 5-HT(1A) gene expression was found in the hippocampus during the prehibernation period and in ground squirrels coming out of hibernation, thus confirming the idea of the hippocampus trigger role in the hibernation. Significant decrease in 5-HT(1A) receptor mRNA level in the midbrain was found in animals coming out of hibernation. There was no considerable changes in 5-HT(1A) receptor mRNA level in different stages of sleep-wake cycle in the frontal cortex. Despite drastically decreased body temperature in hibernating animals (about 37 degrees C in active and 4-5 degrees C in hibernation), 5-HT(1A) receptor mRNA level in all examined brain regions remained relatively high, suggesting the essential role of this 5-HT receptor subtype in the regulation of hibernation and associated hypothermia.