In this study, we compared the effects of xylazine, medetomidine and dexmedetomidine in combination with ketamine on heart rate, respiratory rate, blood gas values, temperature and sedation scores. A total of 30 dogs were evaluated. The dogs were randomly allocated into three anaesthesia groups, each of which included ten dogs. The first group, denoted the xylazine/ketamine group, intravenously received xylazine (0.5 mg/kg) for premedication and ketamine (5 mg/kg) for induction. The second group, the medetomidine/ketamine group, intravenously received medetomidine (10 mu g/kg) followed by ketamine (5 mg/kg). The third group received the dexmedetomidine/ketamine combination. This group intravenously received dexmedetomidine (3 mu g/kg) for premedication and ketamine (5 mg/kg). Heart rate, respiratory rate, oxygen saturation, blood gas parameters and temperature were recorded for all patients immediately before sedation onset (T-0), five minutes after sedation onset (T-1) and five minutes after endotracheal incubation following ketamine injection (T-2). The end tidal carbon dioxide level was recorded at T-2. A significant decrease in heart rate occurred following premedication in all groups. However, the decrease was most marked in the medetomidine/ketamine group. An increase was observed in venous partial pressure of carbon dioxide values at T-2 in the xylazine/ketamine group compared to the medetomidine/ketamine and dexmedetomidine/ketamine groups. The end tidal carbon dioxide levels were higher in the medetomidine/ketamine group than in the other two groups, and oxygen saturation of haemoglobin levels in the same group were found to be lower than in the others. It was determined that none of alpha(2) -agonists, namely xylazine, medetomidine or dexmedetomidine, had superior properties over the others. If medetomidine is used, special care should be taken because of the rapid decrease in heart rate.