Ten women [mean maximal O2 uptake (VO2max), 2.81 l X min-1] exercised for 15 min on a cycle ergometer in the middle of the luteal phase (L) and in the early follicular phase (F) of the menstrual cycle at the same constant work rates (mean 122 W) and an ambient temperature of 18 degrees C. Serum progesterone averaged 44.7 nmol X l-1 in L and 0.7 nmol X l-1 in F. After a 4-h resting period, exercise was performed between 3 and 4 A.M., when the L-F core temperature difference is maximal. Preexercise esophageal (Tes), tympanic (Tty), and rectal (Tre) temperatures averaged 0.6 degrees C higher in L. During exercise Tes, Tty, and Tre averaged 0.5 degrees C higher. The thresholds for chest sweating and cutaneous vasodilation (heat clearance technique) at the thumb and forearm were elevated in L by an average of 0.47 degrees C, related to mean body temperature (Tb(es) = 0.87Tes + 0.13Tskin), Tes, Tty, or Tre. The above-threshold chest sweat rate and cutaneous heat clearances were also increased in L. The mean exercise heart rate was 170.0 beats X min-1 in L and 163.8 beats X min-1 in F. The mean exercise VO2 in L (2.21 l X min-1) was 5.2% higher than in F (2.10 l X min-1), the metabolic rate was increased in L by 5.6%, but the net efficiency was 5.3% lower. No significant L-F differences in the respiratory exchange ratio and postexercise plasma lactate were demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)