The magnitude of improvement in peak oxygen uptake (V(o)(2peak))) and performance to an exercise training regime is related to the V(o)(2) of prior accumulated exercise training bouts. However, it is unclear whether constant rate training (CRT) or interval training (INT) preferentially alters the V(o)(2) of running exercise. Therefore, the purpose of this study was to compare the acute V(o)(2) response to constant, and interval training sessions. Consequently, this study compared the mean average V(o)(2) of 17 moderately trained participants to a 20-min CRT and two different 20min INT treadmill runs. Participants completed three treatments (twice) in random order over 3 weeks. In 1min INT participants completed 10x1min efforts at the velocity corresponding to V(o)(2peak) (V(peak)) interspersed with 10x1min efforts at 0.5V(peak). In the 2min INT, participants completed 5x2min efforts at the V(peak) interspersed with 5x2min efforts at 0.5 at V(peak). In CRT participants ran at a velocity equivalent to the mean velocity of INT (75% V(peak)). Mean average V(o)(2) for 2min INT, 1min INT and CRT were, respectively, 3200+/-661; 3076+/-6041; 2909+/-584mlmin(-1). Both INT sessions resulted in a significantly higher mean average V(o)(2) than CRT. Furthermore, 2min INT resulted in 90% of V(o)(2peak) being exceeded more frequently than 1min INT. We conclude that INT serves as a more potent stimulus for improvement in V(o)(2peak) and subsequent endurance performance than CRT.