This study examined the time course and extent of decrease in peak oxygen uptake (VO2peak) during self-paced exercise in HOT (35°C and 60% relative humidity) and COOL (18°C and 40% relative humidity) laboratory conditions. Ten well-trained cyclists completed four consecutive 16.5-min time trials (15-min self-paced effort with 1.5-min maximal end-spurt to determine VO2peak) interspersed by 5 min of recovery on a cycle ergometer in each condition. Rectal temperature increased significantly more in HOT (39.4 ± 0.7°C) than COOL (38.6 ± 0.3°C; P < 0.001). Power output was lower throughout HOT compared with COOL (P < 0.001). The decrease in power output from trial 1 to 4 was ∼16% greater in HOT (P < 0.001). Oxygen uptake (VO2) was lower throughout HOT than COOL (P < 0.05), except at 5 min and during the end-spurt in trial 1. In HOT, VO2peak reached 97, 89, 85, and 85% of predetermined maximal VO2, whereas in COOL 97, 94, 93, and 92% were attained. Relative exercise intensity (%VO2peak ) during trials 1 and 2 was lower in HOT (∼84%) than COOL (∼86%; P < 0.05), decreasing slightly during trials 3 and 4 (∼80 and ∼85%, respectively; P < 0.05). However, heart rate was higher throughout HOT (P = 0.002), and ratings of perceived exertion greater during trials 3 and 4 in HOT (P < 0.05). Consequently, the regulation of self-paced exercise appears to occur in conjunction with the maintenance of %VO2peak within a narrow range (80-85% VO2peak). This range widens under heat stress, however, when exercise becomes protracted and a disassociation develops between relative exercise intensity, heart rate, and ratings of perceived exertion.