DeGroot DW, Kenney WL. Thermal response to running across the Sahara Desert: data for three men. Aviat Space Environ Med 2008; 79:909–13.Background: There are limited data concerning the physiological responses to long-duration exercise collected under field conditions, and less data under harsh environmental conditions. This paper describes select environmental conditions and physiological responses of three runners attempting to run across the entire Sahara desert over a 111-d period. Methods: The runners started in Saint-Louis, Senegal, and we collected data on 2 d at the start of the expedition. Core temperature was measured via telemetry pill (T_pill), heart rate via Polar monitor, and metabolic rate (M) was estimated from two equations. The Pandolf equation uses movement speed and grade while Berglund's equation predicts M from heart rate and dry-bulb temperature. Data are presented as mean ± SD (range). Results: The runners intermittently ran 8.0 km · h⁻¹ over 6 h during Night (warm-humid) conditions and 6.9 km · h⁻¹ over 7 h 40 min during Day (hot-dry) desert conditions. Mean T_pill was similar for both days (37.8 ± 10.34 vs. 37.82 ± 0.50°C) while range was greater during the day (Day: 36.69–38.91 vs. Night: 37.11–38.48°C). Heart rate was 128 ± 16 (72–156) and 119 ± 17 (75–147) bpm for Night and Day, respectively. M_mean was 299 ± 66 (65–418) W · m⁻² and 364 ± 117 (58–542) W · m⁻² during Night and 239 ± 60 (67–356) and 244 ± 139 (54–464) W · m⁻² during Day, estimated by Berglund and Pandolf, respectively. Conclusions: During Day, the athletes ran slower than during Night, though T_pill was similar, indicative of the greater environmental strain. Mean predicted M was similar between equations, though maximum and minimal values were more extreme and rate-of-change dynamics faster according to Pandolf's equation.