The relation between electroencephalographic sleep parameters and plasma cholinesterase isozymes was examined in a group of 19 unmedicated schizophrenic patients. Rapid eye movement (REM) latency was found to be significantly inversely correlated with isozyme 3 (mainly acetylcholinesterase). The results are discussed in relation to cholinergic involvement in the regulation of REM sleep and in the pathophysiology of schizophrenia.