Insight into the cardiac responses to exercise necessitates an understanding of both physiological data and cardiac dimensional changes. This study was designed to assess cardiovascular alterations during progressive upright cycle exercise in ten healthy 10-12-year-old boys. Doppler echocardiography was used to estimate stroke volume, and 2-dimensional echocardiography was used to evaluate changes in left ventricular systolic and diastolic dimensions. Test-retest reproducibility was high for both techniques. Mean peak stroke index and cardiac index values were 62 +/- 12 ml m(-2) and 11.79 + 2.62 L min(-1) m(-2), respectively. Stroke volume rose by 40% over resting values with early exercise (50 watt work load), but beyond moderate intensities (approximately 50% VO(2)max) little change was seen. The left ventricular diastolic dimension rose slightly at the onset of exercise and then declined slowly. A progressive and more precipitous decline was observed in systolic dimension, resulting in an increase in shortening fraction from 29% to 47%. This was accompanied by a dramatic fall in peripheral vascular resistance from 13.9 to 8.0 units at the onset of exercise. These findings suggest a close matching of increases in heart rate and systemic venous return and imply a significant role of peripheral pump function in the circulatory responses to exercise in children. The cardiac dynamics observed in this study mimic those previously described in adult subjects. Am. J. Hum. Biol. 12:749-757, 2000. Copyright 2000 Wiley-Liss, Inc.