Some pubertal children are susceptible to orthostatic stress but little is known about mechanisms of circulatory adjustment to the posture change in children. We investigated beat-to-beat blood pressure (BP) and heart rate (HR) responses to active standing in 173 schoolchildren, 92 boys and 81 girls, aged 6-18 years (mean age: 13.4 years) using a non-invasive continuous monitoring system (Finapres 2300, Ohmeda). The subjects were divided into four age groups: prepubertal I (7-9 years), prepubertal II (10-12 years), pubertal (13-15 years) and young adult (16-18 years). Supine BP increased and HR decreased with age. At the onset of active standing two older groups showed a significantly larger initial drop than the prepubertal groups (-36 +/- 15 versus -15 +/- 16% reduction for systolic BP and -36 +/- 14 versus -20 +/- 19% for diastolic BP, respectively, p < 0.01). Moreover, the pubertal group had a significantly smaller vasoconstrictor index than prepubertals and two older groups had a significantly more prolonged BP recovery time. In keeping with this the pubertal group most frequently had hypotensive symptoms during active standing. The rise in HR at the peak was higher in two older groups than in prepubertals (34 +/- 9 versus 29 +/- 8 beats/min-1, respectively, p < 0.001), whereas the baroreflex index was almost identical for the four groups. The effect of body proportion on BP responses was not found. There was no significant difference in BP and HR changes in the later stage during 7 min of standing. These results indicated that pubertal children were more susceptible to orthostatic stress, probably due to abnormal BP responses in the initial phase of active standing, which seemed to reflect enhanced cardiopulmonary reflexes and diminished sympathetic activation associated with the age. Moreover, BP reduction at an initial drop of more than 60% or a recovery time of more than 25 s might be postulated to be an abnormal circulatory response.