We investigated the physiological responses in older men to continuous (CEx) and intermittent (IEx) exercise. Nine men [70.4 (1.2) years, VO(2peak): 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% VO(2peak). 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 ( VO(2)), 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 PCO(2). At the same relative intensity (50% or 70% VO(2peak)), IEx resulted in a significantly lower ( P<0.01) VO(2), V(E) and HR than CEx. There were no significant differences ( P>0.05) in VO(2), V(E) and HR measured at the mid point of the three exercise to rest ratios at 50% and 70% VO(2peak). pH and PCO(2) during CEx and IEx at 50% VO(2peak) were not significantly different from rest. CEx performed at 70% VO(2peak) resulted in significant decreases ( P<0.05) in pH and PCO(2). There was a significant decrease ( P<0.05) in pH only during the 60sE:60sR IEx at 70% VO(2peak). Changes in arterialised PCO(2) during the 60sE:60sR, 30sE:30sR and 15sE:15sR at both 50% and 70% VO(2peak) exercise tests were not significant. When exercising at the same percentage of VO(2peak) 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).