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The proportion of muscle fibres innervated by more than one motoneurone (polyneuronal innervation) was measured in the fourth deep lumbrical muscle of young rats following either unilateral deafferentation or spinal cord isolation (cord transection combined with bilateral deafferentation). 2. Unilateral deafferentation at 7 days of age did not affect the subsequent time course of synapse elimination in the developing muscle. 3. Spinal cord isolation at 7 days led to a prolongation of the time course of synapse elimination in the muscle, levels of polyneuronal innervation approaching zero beyond about 30 days (about 20 days in normal animals). The raised levels of polyneuronal innervation in these animals were not associated with significant terminal sprouting at end-plates, but were associated with multiple axonal inputs to end-plates as is found in younger normal animals. This was shown by zinc iodide and osmium staining. Also some muscle fibres had two and sometimes three separate end-plates. 4. Spinal cord isolation in rats 19 or more days old resulted in a reappearance of multiple innervation. This was largely due to ultraterminal sprouting. 5. It is concluded that unilateral deafferentation alone causes no change in the time course of synapse elimination. The multiple innervation caused by total deafferentation and spinal transection is attributed to the lack of activity of these muscles.