The clinical and cardiorespiratory effects of premedication with acepromazine, butorphanol or diazepam in addition to romifidine before induction of anaesthesia with ketamine were studied in 6 horses on 4 random occasions. Administration of romifidine alone or in combination with butorphanol resulted in an increase in arterial blood pressure, accompanied by a significant decrease in heart rate with second-degree atrio-ventricular heart block. Induction of anaesthesia with ketamine returned the heart rate to baseline value, but the arterial blood pressure was significantly increased compared to baseline. Including acepromazine in the premedication prevented the hypertension and bradycardia induced by romifidine. The respiratory rate was slightly decreased after premedication in all groups, but returned to the baseline value after induction of anaesthesia. Mild hypercapnia and significant hypoxaemia were observed during sedation and anaesthesia, reflecting an impairment of pulmonary function. Premedication with acepromazine before sedation with romifidine resulted in a fast induction and good anaesthesia. Inclusion of butorphanol in the premedication resulted in individual variation in the quality of induction and anaesthesia. Addition of diazepam to the sedation with romifidine resulted in good muscle relaxation with a smooth induction and maintenance of anaesthesia and an increased time before the horses responded to noxious stimuli, compared with romifidine and ketamine anaesthesia. All horses reached a standing position at the first attempt, but horses premedicated with diazepam in combination with romifidine showed mild ataxia after recovery.