The influence of vagotomy on resting arterial blood gases (PaCO2 and PaO2) and on the ventilatory responses to hypercapnia and hypoxia was studied in three different species of anaesthetized animals. After bilateral cervical vagotomy resting PaCO2 generally declined. In the dog, where this was studied more fully, resting PaCO2 declined significantly (8-13 +/- 0-67 mm Hg) over several hours, but PaO2 changes were not significant. After vagotomy tidal volume (VT), but not frequency (f), increased with increasing chemical drive. It is concluded that the increase in frequency in response to increased CO2 or decreased O2 is dependent on the vagal mechanisms. Minute volume, fractional inspiratory CO2 (VE, FICO2) and minute volume, fractional inspiratory O2 (VE, FIO2) relationships have been determined. Although vagotomy may not decrease the response of minute volume of ventilation in the lower part of VE, FICO2 curves (mild hypercapnia), it flattened the upper parts of the curves in sheep and rabbits, and to a lesser extent in dogs; it seemed that there was little species difference in this respect. Ventilatory responses to graded hypoxia were close to or slightly higher than pre-vagotomy values throughout the VE, FIO2 curves in all animals.