Clinically significant pseudocholinesterase deficiency is a relatively uncommon disorder, but when present, it presents clinicians with challenges regarding medication administration. This is especially true in cases of patients receiving electroconvulsive therapy (ECT), as the presence of pseudocholinesterase deficiency limits the use of certain muscle relaxants. The authors describe a patient receiving ECT for treatment of his depression, who also possessed an unsuspected pseudocholinesterase deficiency. This was diagnosed after the patient was given succinylcholine, did not spontaneously recover motor function, and eventually required intubation. Subsequent ECT treatments were then managed with an alternative muscle relaxant which was not dependent on pseudocholinesterase for termination of action.