One theory of narcosis postulates that inhaled anesthetics produce general anesthesia by causing the release of endogenous opioid peptides. In the present study, however, the concentration of immunoreactive beta-endorphin-like material (eight patients) or leuenkephalin (four patients) did not increase in cerebrospinal fluid of patients 5 min after induction of anesthesia with thiopental, 2–5 mg/kg and N2O 70%; after an additional 10 min, during which halothane was added; at 5, 15, and 60 min after surgical incision; or after 30 min in the recovery room. Therefore, any contribution of the endorphin system to the production of general anesthesia does not appear to require the release of beta-endorphin.