Eastern red bats (Lasiurus borealis) will move into leaf litter during cold bouts of winter, and because temperatures fluctuate at these roosts, our 1st goal was to quantify winter arousals in response to ambient temperature (T_a). Additionally, we measured changes in metabolism and body temperature (T_b) during hibernation and arousals at various T_a. Using these data, we estimated winter energy budgets. Bats were captured during autumn of 2003 and 2004 in southwestern Missouri and kept in environmental chambers simulating natural conditions. We assessed torpor bout duration using temperature-sensitive data-loggers within environmental chambers at 15°C, 10°C, 5°C, and 2°C. Metabolic rate during torpor was assessed within metabolic chambers at 15°C, 10°C, 5°C, 1°C, and −5°C. Examination of our data suggests that torpor bout duration was not affected by sex and it was inversely related to T_a down to 2°C. Metabolic rate during torpor was not affected by sex but it did vary with T_a. T_b of hibernating bats approximated T_a and the difference between these was greatest at 1°C. Our studies of thermal physiology provide insight of overwintering behaviors of L. borealis, which are apparently less constrained by thermal parameters than in other temperate bat species.