We investigated the physiological responses in older men to continuous (CEx) and intermittent (IEx) exercise. Nine men [70.4 (1.2) years, V_ O2peak: 2.21 (0.20) l min)1; mean (SE)] completed eight exercise tests (two CEx and six IEx) on an electronically braked cycle ergometer in random order. CEx and IEx were performed at 50% and 70%V_ O2peak. IEx was performed using 60sE:60sR, 30sE:30sR and 15sE:15sR exercise to rest ratios. The duration of exercise was adjusted so that the total amount of work completed was the same for each exercise test. Oxygen uptake (V_ O2), minute ventilation (V_ E) and heart rate (HR) were measured at the mid-point of each exercise test. Arterialised blood samples were obtained at rest and during exercise and analysed for pH and PCO2. At the same relative intensity (50% or 70% V_ O2peak), IEx resulted in a significantly lower (P<0.01) V_ O2, V_ E and HR than CEx. There were no significant differences (P>0.05) in V_ O2, V_ E and HR measured at the mid point of the three exercise to rest ratios at 50% and 70% V_ O2peak. pH and PCO2 during CEx and IEx at 50% V_ O2peak were not significantly different from rest. CEx performed at 70% V_ O2peak resulted in significant decreases (P<0.05) in pH and PCO2. There was a significant decrease (P<0.05) in pH only during the 60sE:60sR IEx at 70% V_ O2peak. Changes in arterialised PCO2 during the 60sE:60sR, 30sE:30sR and 15sE:15sR at both 50% and 70% V_ O2peak exercise tests were not significant. When exercising at the same percentage of V_ O2peak and with the total amount of work fixed, IEx results in significantly lower physiological responses than CEx in older men. All results are given as mean (SE).