One hundred and eighteen patients undergoing surgical procedures not requiring immediate tracheal intubation, nor producing visceral reflexes, were pretreated with d-tubocurarine 6 mg, three minutes before the administration of a bolus of succinylcholine (2 mg/kg). Electrocardiographic changes, venous and arterial plasma potassium and calcium levels, CPK changes (12 patients) and appearance of myoglobinuria (35 patients) were followed. Pretreatment with a small dose of d-tubocurarine did not change the overall incidence and pattern of arrhythmias; it did, however, prevent increases in plasma potassium in 90.4 per cent of the patients, mean plasma potassium values remaining below pre-induction levels. The CPK level changed in only one of 12 patients (from 10 to 21 I.U., the normal range being 0 to 34 I.U.). No myoglobinuria was detected in any of the patients tested. A rapid but short-lasting change in the K+/Ca++ ratio did not seem to influence the occurrence of arrhythmias. Pretreatment with a small dose of d-tubocurarine is effective in preventing or decreasing fasciculations, plasma potassium and CPK changes and myoglobinuria described after the intravenous administration of succinylcholine. The already useful role of succinylcholine in our armamentarium can be made safer by pretreatment with a small dose of d-tubocurarine.