Respiratory effects were measured in rats during six hours' anaesthesia with urethane and chloralose. 2. One-hundred min from urethane administration, minute ventilation (VE) was minimal, arterial PO2 was low, arterial PCO2 was high; tidal volume (VT) and respiratory frequency (f) were relatively constant; hypercarbic and hypoxic responses were substantial. 3. Between 100 and 400 min from urethane administration, minute ventilation and frequency increased and became more variable, tidal volume remained relatively constant, arterial PO2 rose to 100 mmHg, PCO2 fell to 37 mmHg; hypercarbic sensitivity increased and hypoxic sensitivity decreased. 4. We conclude that the anaesthetic regime produced initial depression of respiration relative to metabolism but without great loss of respiratory chemosensitivity. The respiratory depression was prolonged by increased dosage with urethane and chloralose. 5. The variations between hypercarbic and hypoxic responses confirm that they operate through separate mechanisms.
