We tested the hypothesis that elevation in heart rate (HR) during submaximal exercise in the heat is related, in part, to increased percentage of maximal O₂ uptake (%O_2 max) utilized due to reduced maximal O₂ uptake (O_2 max) measured after exercise under the same thermal conditions. Peak O₂ uptake (O_2 peak), O₂ uptake, 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 O_2 peak. The conditions involved walking for 20 min at ~33% of control O_2 max in 25, 35, 40, and 45°C followed immediately by measurement of O_2 peak in the same thermal environment. O_2 peak decreased progressively (3.77 ± 0.19, 3.61 ± 0.18, 3.44 ± 0.17, and 3.13 ± 0.16 l/min) and HR at the end of the submaximal exercise increased progressively (107 ± 2, 112 ± 2, 120 ± 2, and 137 ± 2 beats/min) with increasing ambient temperature (T_a). HR and %O_2 peak 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 the increase in %O_2 peak utilized, which is caused by reduced O_2 peak measured during exercise in the heat. At high T_a, the dissociation of HR from %O_2 peak measured after sustained submaximal exercise is less than if O_2 max is assumed to be unchanged during exercise in the heat.