Three hundred and thirty-two patients undergoing electroconvulsive therapy received thiopentone and suxamethonium. In 31 the duration of the apnoea was greater than an assumed upper limit of normal of 240 s. Eight of these patients proved, on biochemical grounds, to be heterozygotes for the atypical gene. The reason why the heterozygous state is rarely present in those who have prolonged apnoea is that the vast majority of those who suffer from this abnormality are only slightly more sensitive to suxamethonium than are normal individuals.