Previous studies have found that selection for endurance running in untrained rats produced distinct high (HCR) and low (LCR) capacity runners. Furthermore, despite weighing 14% less, 7^th generation HCR rats achieved the same absolute maximal oxygen consumption (VO₂MAX) as LCR due to muscle adaptations that improved oxygen extraction and utilization. However, there were no differences in cardiopulmonary function after 7 generations of selection. If selection for increased endurance capacity continued, we hypothesized that due to the serial nature of oxygen delivery enhanced cardiopulmonary function would be required. In the present study, generation 15 rats selected for high and low endurance running capacity showed differences in pulmonary function. HCR, now 25% lighter than LCR, reached a 12% higher absolute VO₂MAX than LCR, P < 0.05, (49% higher VO₂MAX per kg). Despite the 25% difference in body size, both lung volume (at 20 cm H₂O airway pressure) and exercise diffusing capacity were similar in HCR and LCR. Lung volume of LCR lay on published mammalian allometrical relationships while that of HCR lay above that line. Alveolar ventilation at VO₂MAX was 30% higher, P < 0.05, (78% higher, per kg), arterial PCO₂ was 4.5 mmHg (17%) lower, P < 0.05, while total pulmonary vascular resistance was (insignificantly) 5% lower (30% lower, per kg) in HCR. The smaller mass of HCR animals was due mostly to a smaller body frame rather than to a lower fat mass. These findings show that by generation 15, lung size in smaller HCR rats is not reduced in concert with their smaller body size, but has remained similar to that of LCR, supporting the hypothesis that continued selection for increased endurance capacity requires relatively larger lungs, supporting greater ventilation, gas exchange and vascular conductance.