Golden-mantled ground squirrels (Spermophilus lateralis) in the laboratory are most likely to enter torpor when the linoleic acid content of their diet is 33–62 mg/g diet. Diets of free-ranging ground squirrels vary in linoleic acid content throughout summer; however, they often do not have constant linoleic acid content for more than a month. The influence of natural short-term (<1 month) changes in diet levels of polyunsaturated fatty acid (PUFA) on hibernation could not be predicted by previous studies because they involved subjects maintained on diets with the same PUFA content for 7–24 weeks. Hibernation experiments with laboratory feeding were conducted on S. lateralis to determine the influence on torpor of short-term (<1 month) shifts in diet PUFA levels. Reducing diet linoleic acid content for just 19 days immediately before onset of hibernation enhanced torpor of S. lateralis that ingested diets with high–linoleic acid (>62 mg/g) levels during early-summer feeding. But increasing diet linoleic acid levels for 19 days did not increase torpor of squirrels fed diets with low–linoleic acid (<33 mg/g) content during early-summer feeding. Short-term changes in diet PUFA content can thus have a strong influence on hibernation ability and overwinter survival in some circumstances.