Spontaneous ventilation during general anaesthesia leads to respiratory depression and atelectasis. Mechanical ventilation increases tidal volume and eliminates atelectasis. The study material consisted of a total of 20 dogs of different breed, age and gender. Dogs were divided into two groups, consisted of 10 dogs. The first group was established as the spontaneous ventilation (SV) group, while the second group was the mechanical ventilation (MV) group. For induction of anaesthesia, propofol was administered to both groups via intravenous injection at a dose of 6 mg/kg. Blood samples were collected from all dogs in 5 minutes after propofol administration. This period was determined as Minute 0 (T-0). In both groups, inhalation anaesthesia was continued with isoflurane. Venous blood samples were collected from dogs in the SV and MV groups at 15 (T-15), 30 (T-30) and 60 (T-60) minutes. Heart rate, respiratory rate, SpO(2), body temperature and blood gases were monitored. Statistical evaluation of the study was carried out using the Repeated Measures Analysis of Variance method. The results obtained showed that there was no statistically significant difference between the SV and MV groups regarding the examined parameters. However, in the assessments within the group, results obtained from the dogs in the MV group were more reliable from the point of view of the patients remaining stable throughout anaesthesia.