The purpose of this study was to determine whether the tachypneic breathing pattern of constant work rate, heavy exercise (CWE) is unique to CWE or whether it represents the usual pattern of the respiratory control system at high levels of ventilation (VI). We compared breathing pattern in ten healthy subjects (age 20-29 years) during CWE and maximal incremental exercise (MIE) on a bicycle ergometer. Work rate was constant at 76% of maximum work rate in CWE and progressively increased by 25 watts/minute until exhaustion during MIE. Breathing pattern was examined at matched levels of VI equivalent to 80% and about 100% of maximum VI during CWE (97.1 and 121.4 L.min-1, respectively). Exercise duration (mean+standard deviation) was 13 +/- 6 and 12 +/- 1 min during CWE and MIE, respectively (P = NS). Tidal volume (VT) fell by an average of 0.20 L towards the end of CWE, but was maintained relatively high and constant towards the end of MIE. At high, but not lower, matched levels of VI breathing pattern during CWE was significantly more rapid and shallow than that during MIE. The tachypnoea of CWE did not correlate with the progressive rise in VI, oxygen uptake or cardiac frequency during CWE. We conclude that (1) CWE is associated with a tachypneic influence that is absent or less during incremental exercise; this tachypnea is most marked at the end of CWE. (2) The tachypnoea of CWE is not part of a generalized rate accelerating process during CWE. The mechanism(s) underlying the tachypnoea are unclear but it may be related to inspiratory muscle fatigue, pulmonary oedema, and/or altered respiratory mechanics.