Electromyographic analysis of the upper lip region of the rhesus monkey demonstrated that nasal obstruction could induce long-term changes in the discharge of certain fibers of the superior orbicularis oris muscle. Normal animals were compared with 14 monkeys which had their noses blocked by nasal plugs. During the first 6 months of adaptation to oral respiration, the muscle was rhythmically recruited. The EMG pattern suggested that the muscle developed its maximum tension early in its discharge at the onset of inspiration. The total discharge, described as tonic activity, was evident in both control and experimental animals, but tended to occur more often in the experimental animals, especially later in the adaptation period to oral respiration. This suggests that synaptic control of motoneurons of the VII cranial nucleus is modified with the need for using the oral cavity as the anterior portal of the upper respiratory tract, and demonstrates a plasticity in the functional adaptation of one facial muscle.