In eight anesthetized, spontaneously breathing New Zealand rabbits airflow, tidal volume, tracheal and esophageal pressures were measured. Frequency, inspiratory and expiratory durations, respiratory system and lung resistances, and elastances, total postinspiratory muscle pressure and its timing parameters were determined. The measurements were performed: (a) under control condition; (b) continuously, during the first 90 sec of the inhalation of a mixture of 5% CO2 + 95% O2, (carbogen, CI); and (c) 6 min after the start of CI. During CI both lung resistance and elastance decreased, and inspiratory driving pressure and postinspiratory muscle pressure increased. No modification could be detected in either the pattern or the duration of postinspiratory muscle activity. In conclusion, mechanical changes precede and facilitate the ventilatory adjustment to CO2 in rabbits.