Eight well-trained male rowers exercised as hard as possible on a bicycle ergometer for 60 min at an ambient temperature of 18 degrees C. On 1 day (precooling test, PRET) exercise was preceded by a double cold exposure that caused starting body temperatures to be approximately 4.5, 1.0, 0.8, and 0.4 degrees C (mean skin, mean body, tympanic, and esophageal temperatures, respectively) lower than in the control test (CONT). In PRET the mean 1-h work rate (172 W) was 6.8% larger than in CONT (161 W), O2 uptake (Vo2) was 9.6% higher (2.86 vs. 2.61 1 X min-1), and O2 pulse was increased by 5.6% (18.8 vs. 17.8 ml), whereas the sweat rate was 20.3% lower in PRET (1.06 vs. 1.33 mg X cm-2.min-1). No differences in heart rate, efficiency, postexercise blood values of lactate, or acid-base status were demonstrated. It appears that the improved performance in PRET was related to an increased O2 supply to the working muscles. Although plasma levels of total beta H-endorphin immunoreactivity were in the same range under both test conditions, different components of beta H-endorphin immunoreactivity were indicated to exist in PRET and CONT.