The efficacy of nitrous oxide (N2O) inhalation on the elevated orofacial muscle tonus during dental treatment in patients with cerebral palsy (CP) was studied by measuring the mean frequency of electromyographic (EMG) discharge, blood pressure (BP) and heart rate (HR) taking mentally retarded patients (MR) as a control. At a pre-treatment stage, the mean frequency of the EMG was higher in the CP patients than in the MR patients. By forcing the jaw open, the mean frequency of the EMG, BP and HR was elevated significantly in both types of patients. The enhanced level of the mean frequency of the EMG was higher in the CP patients than in the MR patients. After inhalation of N2O, the mean frequency was reduced significantly in both types of patients, however, the reduction rate was higher in the CP patients than the MR patients; BP and HR were not affected. These results demonstrate the selective efficacy of N2O in reducing the orofacial muscle tonus in CP patients during dental treatment probably because of the N2O inhibiting the function of the central nervous system. The present study therefore suggests the beneficial properties of N2O induced sedation performed during dental treatment on CP patients.