The purpose of the present study was to assess the relationship between the rapidity of increased gas exchange (i.e. oxygen uptake VO2) and increased cardiac output (Qc) during the transient phase following the onset of exercise. Five healthy male subjects performed multiple rest-exercise or light exercise (25 W)-exercise transitions on an electrically braked ergometer at exercise intensities of 50, 75, or 100 W for 6 min, respectively. Each transition was performed at least eight times for each load in random order. The VO2 was obtained by a breath-by-breath method, and Qc was measured by an impedance method during normal breathing, using an ensemble average. On transitions from rest to exercise, VO2 rapidly increased during phase I with time constants of 6.8-7.3 s. The Qc also showed a similar rapid increment with time constants of 6.0-6.8 s with an apparent increase in stroke volume (SV). In this phase I, VO2 increased to about 29.7%-34.1% of the steady-state value and Qc increased to about 58.3%-87.0%. Thereafter, some 20 s after the onset of exercise a mono-exponential increase to steady-state occurred both in VO2 and Qc with time constants of 26.7-32.3 and 23.7-34.4 s, respectively. The insignificant difference between Qc and VO2 time constants in phase I and the abrupt increase in both Qc and SV at the onset of exercise from rest provided further evidence for a "cardiodynamic" contribution to VO2 following the onset of exercise from rest.