OBJECTIVE To compare the incidence of arrhythmias in cats receiving either acepromazine or diazepam for pre-anaesthetic medication prior to halothane anaesthesia. METHODS A blinded, randomized clinical study. METHODS Forty-six healthy cats undergoing surgery. METHODS Animals were allocated to one of two groups for pre-anaesthetic medication. Group 1 received diazepam (0.2 mg kg-1). Group 2 received acepromazine (0.02 mg kg-1). The trial drug was administered intramuscularly in combination with buprenorphine (0.01 mg kg-1) 30 minutes prior to induction of anaesthesia with propofol (approximately 5 mg kg-1). Anaesthesia was maintained using halothane: delivered concentration was 1-2% carried in oxygen and nitrous oxide via an endotracheal tube attached to an Ayre's T-piece (with Jackson-Rees modification) breathing system. The incidence of cardiac arrhythmias was determined by continuously monitoring the electrocardiogram from the time of induction until recovery occurred. Demographical group characteristics were compared using analysis of variance. The incidence of cardiac arrhythmias was compared by the Chi squared test. Statistical significance was set at the 5% level. RESULTS The two groups were similar in weight, age, length and type of procedure undertaken. The incidence of arrhythmias was the same in each group (3/23 cases) (p= 1.0). CONCLUSIONS The incidence of cardiac arrhythmias in this study did not appear to be influenced by the nature of pre-anaesthetic medication. CONCLUSIONS The incidence of cardiac arrhythmias under halothane anaesthesia was 13% in this study. Acepromazine did not appear to exert an anti-arrhythmic effect. This may not be the case in a larger scale study.