A study has been made of the factors which determine that the terminals of a foreign flexor nerve at synaptic sites in a unrodele extensor muscle regress on return of the original extensor nerve. The quantal content (m) of the endplate potential (EPP) at flexor nerve terminal synapses, during innervation of a previously denervated extensor muscle, increased in about 8 weeks to reach the same size as at normal extensor nerve terminals; the same time was taken for m to reach normal size at extensor nerve terminals when these reinnervated their own muscle. At flexor nerve terminals, m decreased eventually to zero if the extensor nerve terminals returned within about 6 weeks of synapse formation by the flexor nerve terminals to the same or an immediately adjacent synaptic site to that occupied by these terminals. During this decrease in m at flexor nerve terminals, stimulation of the flexor nerves in the presence of horseradish peroxidase (HRP) showed HRP-labelled flexor nerve terminals present only in those regions of the extensor muscle in which the electrical signs of flexor nerve terminals were observed, indicating that the decrease in m at regressing flexor terminals was accompanied by their vacating synaptic sites. However, flexor nerve terminals failed to regress from the extensor muscle on return of the original nerve supply if the flexor nerve allowed to form synapses for more than about 10 weeks before the return of the extensor nerve.