The effects of 30-h sleep deprivation on cardiorespiratory function either at rest or in exercise were studied in 15 young healthy male volunteers. All subjects performed 1-min incremental exercise tests on a bicycle ergometer until exhaustion and endurance exercise tests at 3/4 of their maximal work rates. Arterialized venous blood samples were withdrawn at rest and during exercise tests to investigate the influence of sleep loss on blood gases. In addition, resting plasma catecholamine levels were also measured in ten subjects. The results showed that 1) resting heart rate, plasma catecholamine levels, and blood pH were decreased while minute ventilation (VI) and CO2 production (VCO2) were increased after 30 h of sleep loss (P less than 0.05), and 2) the maximal exercise performance was reduced by sleeplessness, as indicated by the decreases in the maximal heart rate, peak VI, peak VCO2, peak O2 consumption, and time to exhaustion (P less than 0.05). However, no significant changes in exercise endurance, arterialized venous pH, and PCO2 were found in exercise after sleep deprivation either. We therefore conclude that 30-h sleep loss alters cardiorespiratory function at rest and the ability to perform maximal exercise but not exercise endurance.