Studies were carried out on eighty adult patients to determine whether the technique of ventilation under general anaesthesia significantly altered the action of tubocurarine and gallamine. Tubocurarine, 4 mg/sq.m body surface area, produced a mean adductor pollicis twitch suppression of 67 per cent for a duration (to 90 per cent recovery) of 27.5 minutes. There was no significant difference in response between those patients who were hyperventilated and those whose ventilation was merely assisted. Gallamine, 18 mg/sq.m body surface area, in hyperventilated patients, produced a 68 per cent adductor pollicis twitch suppression for 17 minutes. The same dose given to patients whose ventilation was assisted produced a mean twitch suppression of 40 per cent for a duration of 7.3 minutes. The differing results found in these two groups were statistically significant.