Previous animal models of heat stress have been compromised by methodologies, such as restraint and anesthesia, that have confounded our understanding of the core temperature (T(c)) responses elicited by heat stress. Using biotelemetry, we developed a heat stress model to examine T(c) responses in conscious, unrestrained C57BL/6J male mice. Before heat stress, mice were acclimated for >4 wk to an ambient temperature (T(a)) of 25 degrees C. Mice were exposed to T(a) of 39.5 +/- 0.2 degrees C, in the absence of food and water, until they reached maximum T(c) of 42.4 (n = 11), 42.7 (n = 12), or 43.0 degrees C (n = 11), defined as mild, moderate, and extreme heat stress, respectively. Heat stress induced an approximately 13% body weight loss that did not differ by final group T(c); however, survival rate was affected by final T(c) (100% at 42.4 degrees C, 92% at 42.7 degrees C, and 46% at 43 degrees C). Hypothermia (T(c) < 34.5 degrees C) developed after heat stress, with the depth and duration of hypothermia significantly enhanced in the moderate and extreme compared with the mild group. Regardless of heat stress severity, every mouse that transitioned out of hypothermia (survivors only) developed a virtually identical elevation in T(c) the next day, but not night, compared with nonheated controls. To test the effect of the recovery T(a), a group of mice (n = 5) were acclimated for >4 wk and recovered at T(a) of 30 degrees C after moderate heat stress. Recovery at 30 degrees C resulted in 0% survival within approximately 2 h after cessation of heat stress. Using biotelemetry to monitor T(c) in the unrestrained mouse, we show that recovery from acute heat stress is associated with prolonged hypothermia followed by an elevation in daytime T(c) that is dependent on T(a). These thermoregulatory responses to heat stress are key biomarkers that may provide insight into heat stroke pathophysiology.