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 measured by the slope of V_E-PA_CO2 curves were 1.86 and 1.12 liters/min·mmHg, the difference being statistically significant (p<0.05). During exercise the slope of VE-PA_CO2 decreased from 1.86 to 0.62 in the controls, and from 1.12 to 0.62 in the athletes (p<0.01).The mean slope obtained in the athletes, at rest and during exercise, was about 50-60% of that in untrained subjects (p<0.05).The difference in the V_E-PA_CO2 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.