The aim of this study was to evaluate in indirect way through electrical activity of muscles and nerves the state of motoneuron in different forms of childhood spinal proximal muscular atrophies (SMA). The material comprised 454 cases of SMA type Ia (Werdnig-Hoffmann) types Ib + II (intermediate form) and type III (Kugelberg-Welander form). The quantitative EMG revealed the differences between different forms of SMA's in respect to spontaneous and volitional activity. The most unique phenomenon is the occurrence in SMA Ia form of rhythmic firing of motor unit (MUF) persisting during sleep. The comparison of duration, shape and amplitude of action potentials in acute severe form Ia and mild long-standing form III indicates the inability to increase the motor unit territory in form Ia as opposed to form III where most motoneurons have ability to enlarge their field. It was also stated that in the early stage of SMA the EMG has not only diagnostic but also prognostic value.