BACKGROUND Aging-related changes occur mainly in the cardiopulmonary system and skeletal muscles, bringing about a reduction in physical performance. Consequently, maximal oxygen uptake (VO(2)max) decreases. OBJECTIVE The current study investigated exercise oxygen utilization during maximal aerobic exercise in trained and untrained elderly. METHODS Fifteen trained (59.3 +/- 1.1 years) and 15 untrained (60.1 +/- 1.1 years) elderly underwent a peak cardiopulmonary exercise test on a bicycle ergometer. Arterial O(2 )was defined from echocardiograph and venous oxygen content. RESULTS At rest, trained compared to untrained elderly had significantly (p < 0.05) higher values of end diastolic volume (108.1 +/- 5.8 and 100.7 +/- 6.2 ml, respectively) and stroke volume (68.1 +/- 4.3 and 57.3 +/- 6.5 ml, respectively), while heart rate (68.7 +/- 9.3 and 81.3 +/- 8.2 beats . min(-1), respectively), and mean arterial blood pressure (90.6 +/- 6.9 and 95.4 +/- 7.2 mm Hg, respectively) were significantly lower. At peak aerobic test, the trained elderly, compared to the untrained subjects, achieved significantly (p < 0.05) higher values of end diastolic volume (156.1 +/- 8.2 and 134.1 +/- 7.6 ml, respectively), stroke volume (123.0 +/- 7.9 and 96.0 +/- 4.8 ml, respectively), cardiac output (20.2 +/- 1.5 and 15.0 +/- 1.3 liters.min(-1), respectively) and oxygen uptake (42.1 +/- 2.1 and 31.1 +/- 2.4 ml.kg(-1).min(-1), respectively), while diastolic blood pressure (70.3 +/- 5.6 and 77.5 +/- 4.2 mm Hg, respectively) and total peripheral resistance [4.3 +/- 0.8 and 5.9 +/- 1.41 (dyn.s(-1).cm(-5)).10(-1), respectively], were significantly (p < 0.05) lower. CONCLUSIONS The present study suggests that the differences between trained and untrained elderly in absolute oxygen uptake of the working muscles and peak power output at maximal exercise test are due to physical activity status. The higher aerobic capacity in the trained elderly is related to increased cardiovascular function and to a lesser extent to increased muscle mitochondria concentration and capillarity. Although untrained elderly have reduced maximal oxygen uptake at peak aerobic exercise, intrinsic regulation of mitochondrial function does not seem to be significantly altered because of aging associated physical inactivity. Therefore, untrained elderly can partially compensate for their lower cardiac output by increasing oxygen extraction to levels comparable with those of trained elderly.