The effects of the menstrual cycle on cardiorespiratory variables, blood lactate, and performance were studied in exercising females. Nine healthy subjects, 20--24 yr of age, were investigated in midfollicular and midluteal phases of the menstrual cycle at 33, 66, and 90% of maximum power output (light, heavy, and exhaustive exercise). Occurrence of ovulation was confirmed in all subjects by measurement of progesterone, which increased from 0.6 +/- 0.1 (mean +/- SE) in the follicular to 8.9 +/- 2.2 ng/ml in the luteal phase. There was no difference in heart rate (HR), ventilation, O2 uptake, or CO2 output between the two phases during light and heavy exercise, and there was no difference in HR at exhaustion. Cardiac output measured midway through light and heavy exercise periods was not affected by the phase of testing. Time for which exhaustive exercise could be maintained increased from 1.57 +/- 0.32 in the follicular to 2.97 +/- 0.63 min in the luteal phase (P less than 0.02). Blood lactate was higher in the follicular phase after heavy exercise (6.62 +/- 0.8 vs. 4.92 +/- 0.5 mmol/l) (P less than 0.05) and at exhaustion (8.12 +/- 0.9 vs. 6.76 +/- 0.6 mmol/L) (P less than 0.01). A further study showed no effect of cycle phase on lactate disappearance during exercise. We conclude that while aerobic performance and the cardiorespiratory adaptations to exercise are not influenced by the phase of the menstrual cycle, performance of high-intensity exercise is improved, and lactate production appears to be decreased in the luteal phase when estradiol and progesterone levels are elevated.