This study was designed to determine the factors predicting the post-race rectal temperature in marathon runners. Post-race rectal temperatures of 30 recreational runners (maximum oxygen consumption (VO2max) = 58.3 +/- 5.9 ml O2.kg-1.min-1; mean +/- SD) who completed a 42.2 km marathon at 75.8% (+/- 9.3%) VO2max were measured and related to their levels of dehydration (percent mass loss), their running velocities (km.h-1), and their estimated absolute metabolic rates (1 O2.min-1) for different segments of the 42.2 km race. The influence of certain anthropometric variables was also determined. Percent mass loss during the race (2.5 +/- 1.4%), post-race rectal temperatures (38.9 +/- 0.6 degrees C), and rates of sweat loss (1.0 +/- 0.3 1.h-1) were low. There was no statistical relationship between percent mass loss and post-race rectal temperature. Post-race rectal temperatures were significantly related to the metabolic rates for the full 42.2 km and for the last 21.1 and 6 km of the race, and to the average running velocity for the last 6 km (P less than 0.05 and P less than 0.01). Average sweat rates were related to metabolic rates for 42.2 km and for the last 6 km of the race (P less than 0.05) but were unrelated to running velocity. We conclude that metabolic rate sustained during the latter section of the race, and not the level of dehydration, is the principal determinant of the post-race rectal temperature in marathon runners.