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The EMG of the soleus muscle was recorded with bipolar electrodes chronically implanted in rats aged 10-30 days. Changes in the activity pattern were studied in relation to motor development. The firing pattern and shape of motor units were studied at higher resolution. EMG activity was closely related to motor behaviour and posture. At P11, soleus was only phasically active during movements. Tonic EMG activity appeared with age, and by P16 the activity pattern was similar to that of the adult. The activity level also increased markedly with age. This was paralleled by a change in hindlimb posture. The development of tonic activity precedes the development of dendrite bundles in the motoneuron pool of the soleus muscle, which does not support the hypothesis that bundle formation is causally related to tonic activity in postural muscles. At young ages, the tonic EMG was characterized by a clear oscillation pattern with a frequency ranging from 7 to 20 Hz, the higher frequencies occurring at older ages. This oscillation disappeared around P16, which suggests that it is related to functional development of the stretch reflex. Periods of high frequency firing were observed in the EMG; the highest firing frequencies occurred during locomotion. The maximum firing frequency increased steeply to about 80 Hz at P18. It is suggested that bursts of high frequency firing are related to the development of monoaminergic innervation of the spinal cord. During the first three weeks, motor unit action potentials often showed complex shapes of long duration and considerable spike-to-spike variability. Computer simulation of the summation process of fibre potentials at the electrode showed that the small fibre diameters and high variability in diameter at young ages are major factors with respect to the generation of complex potentials.