Succinylcholine 2-5 mg or atracurium 10-15 mg were given on five separate occasions to a 24-year-old, 64 kg woman homozygous for atypical plasma cholinesterase who was undergoing electroconvulsive therapy (ECT). Atracurium blockade was reversed with atropine, 0.6 mg and edrophonium, 35 mg. Train-of-four stimulation was applied to the ulnar nerve and the force of contraction of the adductor pollicis muscle was recorded. Doses producing 90 per cent first twitch blockade were 2.5 and 15 mg for succinylcholine and atracurium respectively. The onset of action was 6 min for both relaxants, and time to 90 per cent first twitch recovery was 20 min for succinylcholine and 16 min for the atracurium-edrophonium combination. It is concluded that the use of atracurium in these patients does not offer marked advantages over small doses of succinylcholine.