The pattern of innervation and either frequency of miniature end-plate potentials (m.e.p.p.s) or acetylcholine (ACh) sensitivity was examined in individual fibres of the soleus muscle in new-born rats to elucidate the mechanism of elimination of motor nerve terminals. 2. the number of motor nerve terminals innervating a muscle fibre was judged by the difference in latencies of end-plate potentials evoked by stimulation of the ventral roots L4 and L5. 3. At early developmental stages, about 50% of soleus muscle fibres were usually singly innervated by the L5 ventral root and this nerve to fibre relation did not change throughout the developmental days. In contrast to the L5 root, the L4 root innervated all the muscle fibres polyneuronally at day 6. In day 16 or older animals, the remaining half of the fibres were singly innervated by the L4 ventral nerves. Therefore, it is concluded that the elimination of motor nerve terminals in the soleus muscle occurs selectively only for the L4 ventral nerves. 4. The number of polyneuronally innervated fibres was increased, to some extent, within 2 days after transection of the spinal cord at day 12. It is suggested that the increased polyneuronal innervation following the inactivity produced by cordotomy may reflect the recovery of terminals which had lost function only a short time previously during elimination. 5. Though frequency of m.e.p.p.s increased from day 6 to adult age as previously reported, there was no significant difference in the frequencies between singly and polyneuronally innervated fibres at any age. Cord transection at day 12 caused no change in the frequency. 6. ACh sensitivity observed in singly innervated fibres was similar to that in polyneuronally innervated fibres. Furthermore, cordotomy on day 12 did not induce an increase in ACh sensitivity around the end-plate. It is concluded that the elimination of terminals is not causally related to changes in ACh sensitivity. 7. The possible mechanisms of selective elimination of terminals and the process of elimination are discussed.
