Prior heavy exercise (above the lactate threshold, LT) reduces the amplitude of the pulmonary oxygen uptake (V˙O2) slow component during heavy exercise, yet the precise effect of prior heavy exercise on the phase II V˙O2 response remains to be established. This study was designed to test the hypotheses that (1) prior heavy exercise increases the amplitude of the phase II V˙O2 response independently of changes in the baseline V˙O2 value and (2) the effect of prior exercise depends on the amount of external work done during prior exercise, irrespective of the intensity of the prior exercise. Nine subjects performed two 6 min bouts of heavy cycling exercise separated by 6 min baseline pedalling recovery (A), two 6 min heavy exercise bouts separated by 12 min recovery (6 min rest and 6 min baseline pedalling, B), and a bout of moderate exercise (below the LT) in which the same amount of external work was performed as during the prior heavy exercise, followed by 6 min heavy exercise (C). In both tests A and B, prior heavy exercise significantly increased the absolute V˙O2 amplitude at the end of phase II (by ~150 ml min-1), and reduced the amplitude of the V˙O2 slow component by a similar amount. Following 12 min of recovery (B), baseline V˙O2, but not blood [lactate], had returned to pre-exercise levels, indicating that these effects occurred independently of changes in baseline V˙O2. Prior moderate exercise (C) had no effect on either the V˙O2 or blood [lactate] responses to subsequent heavy exercise. The V˙O2 response to heavy exercise was therefore dependent on the intensity of prior exercise, and the effects on the amplitudes of the phase II and slow V˙O2 components persisted for at least 12 min following prior heavy exercise. Experimental Physiology (2001) 86.3, 417-425.