Self-paced time trials have long been used as an indicator of running performance. The purpose of this study was to examine if potential physiological and thermoregulatory differences between treadmill and track running would alter performance in a self-paced 10-km time trial. Ten (n = 10) recreationally trained male distance runners (age: 32 ± 6 years, height: 177 ± 6 cm, body mass: 76 ± 11 kg, % body fat: 14.4 ± 4.5, (Equation is included in full-text article.): 62.2 ± 9.5 ml·kg·min) completed two 10-km time trials in a randomized, counterbalanced order on separate days: 1 on a treadmill at 1% grade (TM), and 1 on a 200-m indoor track (IT). Core temperature, skin temperature, and heart rate (HR) were continuously monitored during the run. The ten-kilometer run time was longer during the IT trial (41.66 ± 5.86 minutes) than during the TM trial (40.10 ± 6.06 minutes; p < 0.001), despite a faster first kilometer in the IT trial (p = 0.029). There were no differences between TM and IT trials in the HR (174 ± 13 b·min and 178 ± 13 b·min, respectively; p = 0.846) or body core temperature (38.6 ± 0.5° C and 38.9 ± 0.5° C, respectively, p = 0.218). Skin temperature was higher in the TM trial (35.1 ± 2.5° C) than in the IT trial (32.7 ± 3.0° C; p = 0.002). These data indicate that performance differences exist between a 10-km time trial performed on a TM vs. an IT, potentially because of differences in pacing strategy or metabolic cost between the 2 conditions.