This study examined the relationship between the slow component of oxygen uptake (VO2) kinetics and muscle electromyography (EMG) during severe exercise in nine young (21.7+/-0.9 yr) and nine older (71.6+/-0.8 yr) men. Oxygen uptake (VO2) and surface EMG activity of the left vastus lateralis muscle were measured during a 7-min square-wave bout of severe exercise on a cycle ergometer. The absolute amplitude of the VO2 slow component was greater and occurred approximately 60 s earlier in the young compared to older subjects. However, the rate of increase in the slow component, expressed as a percentage of the total VO2 response per unit time, was not different between young and older subjects (young: 4.8+/-0.5%.min(-1); older: 4.9+/-0.6%.min(-1)). The mean power frequency (MPF) of the EMG increased significantly during the slow component phase of exercise by 6.4+/-1.0% in the young and by 5.4+/-0.7% in the older group and this rise was not significantly different between the two groups. These results indicate that normal ageing may not alter the VO2 slow component (measured as the rate of increase in VO2) and that this finding may be related to similar muscle fibre recruitment patterns in the two groups during severe-intensity exercise.