In a previous study, bipolar patients were found to be more sensitive than controls to induction of rapid eye movement (REM) sleep by the direct muscarinic agonist arecoline. We have replicated this observation in a new group of patients and controls. However, much overlap between groups was seen. Muscarinic supersensitivity appears to be present in some bipolar affective patients, but the REM induction procedure may not be the optimal method for measuring it.