The cardiovascular effects of a 12-week endurance training regimen were studied among normally active and healthy prepubertal children. Twenty-six 8- to 12-year-old children (20 boys and 6 girls) volunteered and 10 acted as control subjects. The training regimen consisted of distance running for progressively longer periods (from 10 to 35 min) 2 to 3 times per week, with 2 additional sessions per week devoted to running games. Those who were trained ran a cumulative average distance of 95.6 km (58.9 miles). Intensity of work was assessed from running pace and heart rate. The target workout intensity was 75% to 80% of aerobic capacity (Vo2 max). Growth and development accounted for increases in height, weight, body circumferences, and diameters, and fat-free body weight. Heart rate (HR) during submaximal workloads, both running and walking, decreased in the trained group (p less than 0.01) and (p less than 0.05). HRmax did not change, but Vo2 max increased significantly (average 7%) in the trained group but not in the controls. No significant change attributable to training was found for submaximal cardiac output, stroke volume, or arteriovenous oxygen difference. The Vo2 max value before conditioning was a relatively poor predictor of the magnitude of improvement in functional capacity, but those with higher initial Vo2 max logged more cumulative training mileage. It was concluded that prepubertal children respond to an endurance training regimen by improving their running capacity, which is, to a limited extent, associated with increased aerobic capacity.