We tested the hypothesis that elevation in heart rate (HR) during submaximal exercise in the heat is related, in part, to increased percent of maximal oxygen uptake (%VO_2max) utilized, due to reduced VO_2max measured following exercise under the same thermal conditions. Peak oxygen uptake (VO_2peak), and VO₂ and HR during submaximal exercise were measured in 22 male and female runners under four environmental conditions designed to manipulate HR during submaximal exercise and VO_2peak. The conditions involved walking for 20 min at ~33% of control VO_2max in 25, 35, 40, and 45° C, followed immediately by measurement of VO_2peak in the same thermal environment. VO_2peak decreased progressively (3.77±0.19, 3.61±0.18, 3.44±0.17, 3.13±0.16 L^.min^-1), and HR at the end of the submaximal exercise increased progressively (107±2, 112±2, 120±2, and 137±2 bpm) with increasing ambient temperature (T_a). HR and %VO_2peak increased in an identical fashion with increasing T_a. We conclude that elevation in HR during submaximal exercise in the heat is related, in part, to increased %VO_2peak utilized, which is caused by reduced VO_2peak measured during exercise in the heat. At high T_a, the dissociation of the HR from %VO_2peak measured following sustained submaximal exercise is less than if VO_2max is assumed to be unchanged during exercise in the heat.