The possible antitussive effects of dextrorphan (the (+) isomer of levorphanol) and phencyclidine (PCP) were compared to well known antitussive properties of dextromethorphan in the post-halothane anesthetized decerebrate cat in which cough was elicited by direct electrical stimulation of the cough center. Dextrorphan, when injected i.a. (0.05-0.32 mg kg-1) or i.v. (1 to 3 mg kg-1), PCP i.a. (0.1-0.32 mg kg-1) or i.v. (1.0 mg kg-1) had no effect on electrically elicited cough. After i.v. administration, dextrorphan caused a variable effect on respiration but did not have any respiratory effect with i.a. administration of the drug. PCP injection i.a. at 0.32 mg kg-1 severely inhibited respiration though coughing could still be elicited. But i.v. administration of 1.0 mg/kg-1 suppressed both cough and respiration for several hours. Dextromethorphan inhibited cough upon both i.a. and i.v. injection. The mean effective i.a. dose was 0.063 mg kg-1. A ten times higher dose was necessary (0.65 mg kg-1) for cough suppression by the i.v. route. It is concluded from the i.a./i.v. ratio that dextromethorphan has specific central antitussive activity not possessed by dextrorphan and PCP.