Increased sensitivity to succylcholine may be due to inherited variants of serum cholinesterase (acylcholine-acylhydrolase, E.C.3.1.1.8). Four different allelic genes at locus E1 controlling human serum cholinesterase have been identified: the normal gene Eu1, the atypical gene Ea1, the fluoride-resistant gene Ef1, and the silent gene Es1. These 4 genes give rise to 10 possible genotype combinations. The sensitivity of some of these genotypes to succinylcholine, as, for instance, homozygotes for the atypical enzyme Ea1 Ea1, is well documented. However, the relation between some of the rarer genotypes and the sensitivity to succinylcholine is less well clarified. The present report represents the 1st case in which a peripheral nerve stimulator has been used to substantiate the increased succinylcholine sensitivity of a patient heterozygous for the silent and the fluoride-resistant gene, and the 2nd published case of prolonged apnea following succinylcholine in a patient carrying the Ef1 Es1 genotype for abnormal serum cholinesterase.