OBJECTIVE: To assess heart rate (HR), intestinal body temperature (T(intest)), and hydration status changes and relationships in 12 participants in a 3-day trail run. DESIGN: Descriptive field study. SETTING: : Three Cranes Challenge trail run, in Karkloof, KwaZulu-Natal, South Africa. PARTICIPANTS: Twelve (5 men and 7 women) amateur runners. INTERVENTIONS: Trail run of 95 km divided into 3 stages: elevation gains on the 3 days, 1020, 1226, and 680 m, respectively. MAIN OUTCOME MEASURES: Changes in HR, T(intest), serum osmolality, and body mass. RESULTS: Environmental conditions were consistently mild (ambient temperature range, 11.5-22.8°C; maximum relative humidity range, 95%-97%), average running speed varied from 9.00 to 5.14 minutes/km, and distance covered in the 3 stages ranged from 32 (stages 1 and 3) to 40 km (stage 2). Mean HR ranged from 134 to 171 beats per minute in the 12 athletes during the trail events and averaged at 150 beats per minute, whereas T(intest) ranged between 36.1 and 40.2°C. The correlation between maximum T(intest) and percent age-predicted maximum HR (n = 12) was significant (R = 0.58; P < 0.05), whereas the correlation between maximum T(intest) and serum osmolality or body mass did not reach significance (R = 0.16, 0.13; P > 0.05). CONCLUSIONS: This study provides evidence in support of the contention that maximum T(intest) is more closely related to metabolic rate during trail running than percent dehydration. The findings do not support an increase in core body temperature with a change in serum osmolality or body mass.