Preoperative administration of atropine was evaluated during induction of halothane anaesthesia with two administrations of suxamethonium 1 mg/kg body weight, 5 min apart. Sixty-eight healthy, adult patients were studied. They were divided into five groups according to dose and route of administration of atropine. ECG was continuously monitored. Serum potassium, pH, PaCO2, PaO2 and standard bicarbonate were measured at appropriate intervals. It was found that neither atropine 0.01 mg/kg body weight given intramuscularly 1 h beofre the anaesthesia nor atropine 0.01 mg/kg body weight given intravenously 5 min prior to induction protected against serious bradycardias (defined as heart rate below 20 beats per minute) following the second dose of suxamethonium. No serious brady-arrhythmias were seen in patients given either a combination of intramuscular and intravenous atropine in the above-mentioned doses or in patients given atropine 0.015 mg/kg body weight intravenously 5 minutes prior to induction. However, a decrease in heart rate to around 40-50 beats per minute occurred in some of these patients. Furthermore, these large doses of atropine caused an increase in heart rate during induction to more than 120 beats per minute in about 50% of the patients and to more than 140 beats per minute in about 25% of the patients. Our results suggest that preoperative administration of atropine does not protect against serious brady-arrhythmias following a second dose of suxamethonium, unless doses of atropine are used which cause tachycarida of considerable degree.