The present study was conducted to evaluate the effects of dextromethorphan and codeine on the elements of the coughing motion under the condition of upper respiratory inflammation. Coughs were induced by microinjection of citric acid into the upper trachea of conscious and unrestrained guinea pig. Cough numbers were counted and cough expiratory peak flows, expiration time and inspiratory peak flows were measured by a one-chambered whole body plethysmograph. The percent inhibitions of the values by intraperitoneal administration of the antitussives were assessed. Cough numbers were reduced by both drugs. But cough expiratory peak flows were reduced more effectively by administration of dextromethorphan. Cough expiration time and inspiratory peak flow were not affected significantly by either dextromethorphan or codeine. These results suggest that there are differences between dextromethorphan and codeine in terms of effects to cough pattern generator. It would be important to assess the percent inhibition of cough expiratory peak flow as well as those of cough numbers when evaluating the efficiency of an antitussive
