This study investigated whether the progressive rise in heart rate (HR) and fall in stroke volume (SV) during prolonged, moderate-intensity exercise (cardiovascular drift) in a . thermoneutral environment is associated with a reduction in VO2max. CV drift was measured on . nine moderately-trained women (age = 23.31.8 yr, VO2max = 43.05.5 ml/kg/min) between 15 .. and 45 min of cycling and treadmill walking at 60% VO2max, with VO2max measured immediately . thereafter without cessation of exercise. During a separate trial, VO2max was measured after 15 . min submaximal exercise, so that change in VO2max that occurred between 15 and 45 min could be associated with CV drift. This protocol was performed during two different modes of exercise, cycling and walking. There was no significant effect of exercise mode on the . maginitude of CV drift or change in VO2max. In cycling and walking, respectively, a 5.5% increase in HR (15014 vs 15817 bpm, p<0.05) and a 10% decrease in SV (7710 vs 689 ml, . p<0.05) between 15 and 45 min was associated with a 5% reduction in VO2max (2.700.40 vs . 2.520.57 l/min, p<0.05). There was a significant relationship between .HR and . %VO2max . (r=0.47, SEE=0.32 l/min, p<0.05). Submaximal VO2 was unchanged over time (p<0.05), but . %VO2max increased 5.2%. It is concluded that CV drift, characterized by the rise in HR and decline in SV during prolonged, moderate-intensity exercise, is associated with a decrease in . VO2max during walking and cycling. Increased HR reflects increased relative metabolic intensity during prolonged cycling and walking. This study supports the validity of using HR as an indicator of relative metabolic intensity during prolonged cycling and walking when CV drift occurs.