The protection against bradycardia afforded by atropine given intravenously just prior to a second dose of suxamethonium during halothane inhalation anaesthesia was studied in 100 healthy, adult patients randomly allocated to one of five groups characterized by dosage of atropine. ECG monitoring was continuous, and regular determinations were made of serum potassium, PaCO2, PaO2 and blood pressure. Slowing of the heart rate was seen in more than 50% of patients in each group, but bradycardia (heart rate less than 60 beats/min) was seen only in patients receiving the lowest dose of atropine--0.0075 mg/kg. In the other four groups (atropine 0.01 mg/kg-0.02 mg/kg), bradycardia prophylaxis was effective, but at the cost of serious ventricular arrhythmias in 15% of the patients. The incidence of these arrhythmias seemed to increase with increasing atropine dosage. Marked tachycardia was also seen. Because of the incidence of side effects in this and other studies, no absolute recommendation can be made about suxamethonium bradycardia prophylaxis during halothane inhalation anaesthesia, but our present experience suggests that atropine in a dose not exceeding 0.01 mg/kg, given 30 s prior to a second dose of suxamethonium is best.