ABSTRACT.Twelve girls eight to thirteen years of age were studied before and after six and twelve weeks of cross‐country running. Eight untrained controls were tested twice at six week intervals and weighed after 12 weeks. Oxygen consumption, ventilation, and heart rate were measured during continuous increment treadmill runs to maximum. Mean maxVo2increased by 18 percent at 6 weeks and 26 percent at 12 weeks above pre‐training values. Max &OV0312;Ewas unchanged. Heart rates at submaximal and maximal work loads showed moderate but consistent declines in both runners and controls. Three runners lost 0.7 to 2.0 kg during the season, but the rest gained an average of 0.6 kg. Hence, these pre‐adolescent girls were capable of adapting to endurance training in a fashion similar to adults. No detrimental effects were observed save unexplained weight loss in two runners.