KREIDER, R. B., T. BOONE, W. R. THOMPSON, S. BURKES, and C. W. CORTES. Cardiovascular and thermal responses of triathlon performance. Med. Sci. Sports Exerc, Vol. 20, No. 4, pp. 385- 390, 1988. Triathletes typically train each triathlon event separately. Therefore, to determine the cardiovascular and thermal differences between training and triathlon performance, nine male triathletes performed a simulated 75-min (40 km) control bike and a 40-min (10 km) control run at 70% of maximal oxygen uptake. Control data were compared to data derived from a simulated triathlon (0.8-km swim, 75-min bike, and 40-min run). Results demonstrated that prior swimming significantly decreased (P<0.05) triathlon cycling work output (191 ± 4.2 to 159 ± 7.6 W) producing mean differences (P<0.05) in oxygen uptake (3.18 ± 0.1 to 3.01 ± 0.1 l·min−1), ventilation (84.7 ± 4 to 80.4 ± 4.2 l·min−1). stroke volume (128 ± 7.1 to 118 ± 3.5 ml·min−1), cardiac output (20.7 ± 1.2 to 18.9 ± 0.8 l·min−1), mean arterial pressure (105 ± 3.8 to 96 ± 7.9 mm Hg) and rectal temperature (38.2 ± 0.2 to 38.4 ± 0.3°C). Triathlon running, while performing identical control work output, elicited significant increases (P<0.05) in oxygen uptake (3.41 ± 0.1 to 3.85 ±0.1 l·min−1), ventilation (91.3 ± 3.3 to 104.2 ± 2.8 l·min−1), heart rate (161 ±3.1 to 174 ± 3.6 beats·min−1),arteriovenous oxygen difference (15.3 ± 0.2 to 17.2 ± 0.3 ml-100 ml−1) and rectal temperature (38.3 ± 0.2 and 39.2 ± 0.3°C) with significantly lower (P<0.05) stroke volume (138 ± 2.4 to 129 ± 3.6 ml·min−1) and mean arterial pressure (102 ± 11.2 to 89 ± 5.5 mm Hg). Results demonstrate that triathlon performance elicited cardiovascular and thermal adjustments not experienced when performing the events independently. Findings appear to be a result of thermoregulatory and cardiovascular adaptations combined with a decrease in mechanical efficiency observed during prolonged triathlon performance.