A controlled trial was conducted to assess suitability of combinations of medetomidine and ketamine for the ovariectomy of cats, to investigate the possible side effects, and to compare medetomidine/ketamine with a combination of xylazine and ketamine. Three hundred and thirty-seven cats were submitted to surgery; 100 were anaesthetised with 80 micrograms/kg medetomidine and 5 mg/kg ketamine, 137 with 80 micrograms/kg medetomidine and 7.5 mg/kg ketamine, and 100 were anaesthetised with 1 mg/kg xylazine and 10 mg/kg ketamine. The combinations were injected intramuscularly in the same syringe. The anaesthesia provided by the medetomidine/ketamine combinations was characterised by good muscle relaxation, good analgesia and minimal side effects. The only difference between the two doses of ketamine was the length of the period of anaesthesia. The advantages of the medetomidine/ketamine combination in comparison with xylazine/ketamine were the need for a lower dose of ketamine, a longer duration of action and better analgesia. Similar side effects were observed with both medetomidine/ketamine and xylazine/ketamine combinations.