We hypothesised that initiating heavy-intensity exercise from an elevated baseline metabolic rate would result in slower Phase II O 2 uptake (V˙O2) kinetics and a greater overall ‘gain’ in V˙O2 per unit increase in work rate. Seven healthy males performed a series of like-transitions on a cycle ergometer: (1) from light exercise to ‘moderate’ exercise (80% of the gas exchange threshold, GET; L→M); (2) from light exercise to ‘heavy’ exercise (40% of the difference between GET and V˙O2 peak; L→H); (3) from moderate exercise to heavy exercise (M→H). The Phase II time constant (τ) was significantly (P<0.01) greater in the M→H condition (48±11s) compared to the L→M and L→H conditions (26±6s versus 27±4s, respectively). Moreover, the end-exercise ‘gain’ values were significantly different between the three conditions (L→M, 8.1±0.7mLmin −1 W −1 ; L→H, 9.7±0.4mLmin −1 W −1 ; M→H, 10.7±0.7mLmin −1 W −1 ; P<0.05). This ‘non-linearity’ in the pulmonary V˙O2 response to exercise might be attributed, at least in part, to differences in the metabolic properties of the muscle fibres recruited in the abrupt transition from a lower to a higher work rate.