The effects of halothane, isoflurane and sevoflurane snaesthesia on end tidal CO2 (ETCO2) and arterial oxygen saturation (SpO(2)) were investigated in dogs. The anaesthesia was induced with propofol 2 mg/kg IV; maintained with halothane, isoflurane or sevoflurane in oxygen in the concentrations 1.5%, 2%, 3%, respectively. Intermittent positive pressure ventilation (tidal volume, 15 ml/kg; respiration rate, 12/min) was started immediately after intubation and the anaesthesia was continued for 60 minutes. The end-tidal carbon dioxide concentration was measured continuously at the proximal end of the tracheal tube using CO2 monitor. SpO(2) readings were obtained with a pulse oximeter whose probe was placed on the tongue. End-tidal CO2 during halothane anaesthesia was lower than isoflurane and sevoflurane anaesthesia. SpO(2) in dogs of the halothane group was less than in dogs of other two groups. Isoflurane or sevoflurane anaesthesia appear to be safer than halothane anaesthesia in dogs.