It is well established that a normally innervated, uninjured mammalian skeletal muscle does not accept further innervation from a foreign nerve transplanted on to the muscle's extrasynaptic region. Because synapse formation under these circumstances would require the induction of new postsynaptic specialisations, it is not clear whether foreign axons could make connections if they were allowed instead to compete for existing synaptic sites. The capacity of individual endplates to accept multiple inputs has in fact been demonstrated during both normal development and reinnervation of adult mammalian muscle, that is, in conditions involving interactions among recently formed synapses. We report here that even in adult muscles whose normal innervation remains fully intact, a similar susceptibility to further innervation can be expressed following appropriate placement of a transplanted foreign nerve. Moreover, the establishment of foreign synapses can lead to suppression of original inputs.