The relationships between age and cardiorespiratory and metabolic adjustments during cycle ergometer exercise and treadmill walking were investigated at 30%, 60%, 80%, and maximal aerobic power (VO2max). The subjects were 18 females between the ages of 17-40. Tr-admill walking resulted in an 8 percent greater maximal aerobic power than that observed during cycle ergometer exercise. Higher lactate concentrations, respiratory exchange ratios and lower heart rates during the cycle ergometer test supported the possibility of localized fatigue from anaerobic metabolism as a limitation to aerobic power potential. Contrary to previous observations in males, female subjects die not demonstrate a lower stroke volume or higher heart rate during submaximal cycling compared to treadmill walking. At exercise intensities above 30% Vi2max, plasma bicarbonate and pH were consistently lower following exercise on the cycle ergometer. Respiratory compensation as calculated from "excess ventilation," delta VE/delta HCO3-, was approximately 4 1 min-1/mEq 1-1. Compensation was found to be independent of the type of exercise but linearly related (r=0.99) to the resultant metabolic acidosis. It was concluded that cardiovascular responses to equivalent cycle ergometer and treadmill exercise are similar; however, occurrence of a greater metabolic acidosis during cycle ergometer exercise suggests that a smaller muscle mass is used.