Heat shock protein 70 (HSP70) gene expression was studied in a seasonal hibernator, the diurnal ground squirrel, Spermophilus lateralis. RNA transcripts of 2.7 and 2.9 kb hybridizing to an HSP70 cDNA were expressed in both brain and peripheral tissues of pre-hibernation euthermic animals; higher levels of expression were observed during the day than during nighttime samples. A decline in the expression of both transcripts occurred in all tissues examined during hibernation that remained low throughout the hibernation season, including the interbout euthermic periods and regardless of time of day. Quantitative comparisons showed pre-hibernation nighttime HSP70 expression to be as low as that observed during hibernation, despite the drastic increase in metabolic state and nearly 30 degrees C difference in body temperature. In contrast to HSP70, some mRNAs, such as beta-actin and HSP60, remained relatively constant, while others, such as glyceraldehyde 3-phosphate dehydrogenase, increased in specific tissues during the hibernation season. These results indicate that the expression of a highly conserved gene involved in protection from cellular stress, HSP70, can vary with an animal's arousal state.