Ventilatory responses to CO2 during rest and exercise were studied in 10 marathon runners and 14 untrained subjects by the rebreathing method. The average responses of the untrained subjects and athletes at rest as meausred by the slope of VE-PACO2 curves were 1.86 and 1.12 liters/min-mmHg, the difference being statistically significant (p less than 0.05). During exercise the slope of VE-PACO2 decreased from 1.86 to 0.62 in the controls, and from 1.12 to 0.62 in the athletes (p less than 0.01). The mean slope obtained in the athletes, at rest and during exercise, was about 50-60% of that in untrained subjects (p less than 0.05). The difference in the VE-PACO2 curves between the athlete and the untrained group may be due to a reduced exitability of the respiratory center and/or by a reduced input signal.