The purpose of this study was to investigate whether or not the ventilatory and circulatory responses at the onset of voluntary exercise and passive movement, especially at the initial stage (phase I), in children are the same as in adults. Ten pre-teenage male children and ten adult men participated in this study. Voluntary exercise and passive movement were performed in a sitting position for about 20 s. Both the exercise and the movement consisted of flexion-extensions of the lower leg from a vertical to horizontal position, either voluntarily or passively, with a frequency of about 60 x min(-1). Inspiratory minute ventilation (V1), tidal volume (VT), respiratory frequency, partial pressure of end-tidal CO2 and O2, heart rate (fc) and mean blood pressure (BP) before, during and after exercise or movement were measured using breath-by-breath and beat-to-beat techniques. Cardiorespiratory responses at the onset of voluntary exercise and passive movement were compared with the relative change (delta), which was estimated from the value at rest (100%). In the present study, it was found that: (1) the V1 during voluntary exercise were significantly lower in the children, mainly due to lower deltaVT; (2) the delta(f)c during voluntary exercise was almost the same in both groups, while deltaf(c) was significantly lower in the children during the last part of passive movement; (3) in the voluntary exercise and passive movement, the BP in the children was increased a little or remained close to the value at rest, while it was significantly decreased in the adults. As a result, there were significant differences in deltaBP between the two groups during voluntary exercise. These results suggest that the cardiorespiratory responses at the onset of voluntary exercise and passive movement may be modified during the growth process.