The purpose of this study was to determine the influence of hydration status on pacing of trail runners in the heat (wet bulb globe temperature = 26.2 +/- 1.8 degrees C). A randomized, crossover design was used and the participation occurred within a 2-week period. Seventeen competitive, well-trained distance runners (9 men, 8 women, age 27 +/- 7 years, height 171 +/- 9 cm, weight 64.2 +/- 9.0 kg, body fat 14.6 +/- 5.5%) completed the study. Subjects started maximum effort trials that were either hydrated (HYR) and dehydrated (DHR). Each trial subjects ran three 4-km loops with a 4-minute rest between loops. Significance was set at p < or = 0.05. The DHR had a significantly greater body mass loss at the pre- and posttrial time points (-2.05 +/- 1.25 and -4.3 +/- 1.25%, respectively) vs. HYR (-0.79 +/- 0.95 and -2.05 +/- 1.09%, respectively). Subjects ran the 12 km faster (p < 0.001) in HYR (3,191 +/- 366 seconds) vs. DHR (3,339 +/- 450 seconds). Differences between fastest and slowest loops during HYR (54 +/- 40 seconds) were significantly smaller than DHR (111 +/- 93 seconds; p = 0.041). Additionally, loop times were slower for loop 1 (HYR 1,039 +/- 116 seconds vs. DHR 1,071 +/- 123 seconds; p = 0.028), loop 2 (HYR 1,066 +/- 123 seconds vs. DHR 1,105 +/- 148 seconds; p = 0.01) and loop 3 (HYR 1,081 +/- 132 seconds vs. DHR 1,168 +/- 189 seconds; p = 0.003) when dehydrated. Percent of the race completed by loop as calculated by finishing time was significantly different at loop 2 between HYR (33.6 +/- 0.36%) and DHR (33.1 +/- 0.35%, p = 0.002) and loop 3 (33.8 +/- 0.75% vs. 34.9 +/- 1.35%, respectively, p = 0.01). Total variation from the mean pace for the duration of the HYR compared to the DHR approached significance (p = 0.064). Average percent of variance approached significance between trials (p = 0.057).