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A long-held assertion has been that nocturnality is an escape mechanism for many nocturnal desert rodents because of limited tolerances to heat. To test this claim, we used a treadmill to examine the tolerances to high ambient temperatures (T-a’s) of one subspecies of desert rodent, Merriam’s kangaroo rat, Dipodomys merriami merriami, from contrasting environments. We simultaneously measured body temperature (T-b), evaporative water loss, and metabolic rates at an ecologically relevant speed (0.6 km h(-1)) at different ambient temperatures (T-a = 25 degrees -42.5 degreesC). We hypothesized that kangaroo rats from a more xeric site would have greater abilities to remain active and maintain stable T-b than those from a more mesic site, but mesic- and xeric-site animals had comparable tolerances and were active until T-b = 42 degreesC. At T-a = 42.5 degreesC. however, T-b of mesic-site animals increased more quickly than in xeric-site animals. Although most animals could not run more than 18 min at T-a = 42.5 degreesC, most could run at T-a = 40 degreesC for at least 30 min. Benefits of nocturnality for this species may reside more in purposes of water conservation and avoidance of predation and less on the direct regulation of T-b, as T-b is more labile than commonly thought.