Organoleptic and gas chromatographic methods were employed to establish the threshold of odor objectionability of methylmercaptan and hydrogen sulfide and to assess the relative effectiveness of different oral hygiene measures to reduce the malodor to acceptable levels. The study showed that methylmercaptan and hydrogen sulfide concentrations below 0.5 ng. and 1.5 ng., respectively, are considered nonobjectionable. Gas chromatographic analyses indicate that these concentrations were exceeded in the early morning mouth air samples of approximately 50 per cent of the adult population studied. In these instances, methylmercaptan and hydrogen sulfide occurred in sufficiently high concentrations to account for the malodor. Brushing studies suggest that the early morning malodor arising from the oral cavity can be controlled by proper oral hygiene. The tongue was the major source of both offending compounds in the persons studied. Methods that involved cleansing of the dorsoposterior surface of the tongue caused the most pronounced reductions of both compounds. Since methylmercaptan was found to be more objectionable and to exhibit a lower threshold of objectionability, it was more difficult to reduce to acceptable levels.