A 15-year-old, fractious, weighing 470 kg, Achal-Tekkiner stallion was referred for elective castration. Pre-anaesthetic clinical evaluation revealed tachycardia with a heart rate (HR) of 66 bpm and diastolic left side heart murmur II/VI grade. Lung auscultation was unremarkable. Acepromazine (0.03 mg/kg) was administered intramuscularly to calm the horse for reassessment. Thirty minutes later, HR was 120 bpm. An electrocardiogram (ECG) showed monomorphic ventricular tachycardia. A bolus of Ringer lactate, 10 ml/kg over 30 minutes, and magnesium, 27 g diluted in 1 L 0.9% sodium chloride (NaCl), was administered intravenously. As arrhythmia persisted, a constant-rate infusion of lidocaine, 50 mcg/kg/h, was started. The horse was monitored with telemetry ECG. The arrhythmia resolved after 24 hours and echocardiography showed mild tricuspid, aortic and pulmonary insufficiency. Anaesthetic risk was considered to be acceptable and the procedure was performed without complications. The horse was discharged after another 24 hours without detectable ECG abnormalities.