The maximal aerobic capacity and the aerobic-anaerobic transition were analyzed on 14 triathletes performing an incremental work load on a bicycle ergometer and on horizontal or inclined treadmills. To compared the cardiorespiratory responses between cycling and running, the subjects were divided into 2 groups of 7 with similar aerobic capacity determined from cycle runs. The first group ran on horizontal treadmill while the second group performed similar exercise on inclined treadmill at constant grade (1.5%). Heart rate was recorded by electrocardiogram. Oxygen uptake (VO2), CO2 production (VCO2), respiratory frequency, and pulmonary ventilation were monitored at 30 s intervals through a Rudolph valve connected to a calibrated Oxycon V. Tidal volume, respiratory exchange ratio, equivalent O2 and CO2 were calculated from on-line computer. Aerobic and anaerobic thresholds were determined by a non-invasive method from pulmonary ventilation curves. The results showed that maximum oxygen uptake (VO2max) did not differ between the 2 types of ergometers. Pulmonary ventilation, heart rate and VO2 recorded at aerobic and anaerobic thresholds depended on the mode of exercise and reached the highest values on inclined treadmill. The amount of muscle mass, the type and the distribution of active motor units involved in each exercise test might be at the origin of these differences. This indicates that, when assessing a training program from anaerobic threshold values, it is necessary to take into consideration the type of ergometer used and the protocol performed.