The aim of the study was to investigate the motor unit action potential (MUAP) of the lateral pterygoid muscle and bioelectrical activity of masticatory muscles in temporomandibular joint (TMJ) dysfunction patients undergoing splint therapy. The study included patients aged 18—30 with TMJ pain dysfunction (clinical group). Control group enrolled 25 healthy volunteers. Electromyography was monitored before and after 1, 3, 6 and 12 months from the beginning of therapy with occlusal splints. Increased functional activity of all masticatory muscles was registered in the resting phase in patients with TMJ dysfunction. Duration and amplitude of the MUAP of the lateral pterygoid muscle in the functional activity phase were less on the TMJ healthy side. Amplitude of a bioelectrical activity of masseter and temporal muscles at maximal voluntary contraction was less on the TMJ pain side. High correlation were observed between indicators of the lateral pterygoid muscle on the TMJ healthy side and masseter muscle on the TMJ pain side. After 6 months of the splint-therapy stabilization of amplitude of masseter muscle was noticed; at the same time the lateral pterygoid muscle began it’s reparation. After 12 months the start of the treatment the duration and amplitude of the MUAP of the lateral pterygoid muscle symmetry was received and measured up control level.