Aim of the study was to assess electrophysiologic characteristics of masticatory muscles, right and left temporalis and right and left masseter, in nasal and oronasal breathing modes. The study was conducted on the group of 30 female volunteers who were referred to the clinic for orthodontic problems. Conflict of interests was ruled out. The research was carried out within a frame of population-based study. In subjects with nasal breathing electromyographic indicators fall within standardized range. In the group of subjects with oronasal breathing electrical activity of masseter and temporalis muscles is asymmetrical. Asymmetry between sides and unevenness between muscle pairs during normalized muscle activity may result from functionally unstable occlusion. When teeth clench on a cotton roll, dentoalveolar proprioceptive signal is too weak to affect electromyographic indicators offering possibility to obtain data solely on muscle activity during maximal voluntary contraction. Indicators of the degree of mean muscle contractility recorded as a result of standardized maximal voluntary contraction show that, in nasal breathing, muscle activity is homogenous and symmetric in the right and left masseter and temporalis muscles, while, in oronasal breathing, it becomes more dissociated with electrical activity - more pronounced in temporalis muscles. Electrophysiologic activity and, consequently, contractility of both temporalis and masseter muscles are reduced compared to subjects with nasal breathing which must be caused by recruitment of less excitable motor units. Results yielded by the study suggest the significance of the state of neuromuscular balance of the masticatory apparatus in the assessment of orthodontic status and possibility of applying electromyographic indicators to estimate the degree of orthodontic dysfunction and develop individualized treatment plans. Study continues in this respect.