SUMMARY1. Drug-induced membrane current fluctuations were analysed at frog ectopic neuromuscular junctions formed de novo by somatic motoneurones and by preganglionic autonomic neurones of the vagus nerve, and at denervated end-plates reinnervated by the vagus nerve.2. At ectopic motor end-plates, the mean open time of the ion channels activated by ACh is TACh = 1P8+0'1 msec (S.E.) at -70 to -90 mV and 15-18 'C. Carbacholand suberyldicholine-induced channels have average open times TCarb = 1P0 + 0-1 msec (-80 mV, 6-8 TC) and TSub = 3-3+041 msec (-80 mV, 16-18 TC).3. The conductances y of single channels induced by ACh, carbachol and suberyldicholine are: YACh = 210+ 11 pS (+S.E.), YCarb 146+12 pS and YSub = 22 1 + 1-2 pS.4. The mean open times T of the channels is prolonged as the membrane is hyperpolarized. Their voltage dependence is similar to that of normal end-plate channels.5. The average open time of ACh-induced channels at ectopic vagus junctions is r = 1-6 + 0-2 msec (S.E.) at -70 to -80 mV and 14-15 'C. At denervated motor end-plates reinnervated by vagal neurones, the mean channel open time is T = 1P5 + 0-1 msec at -80 mV and 14-18 'C.6. At all vagus junctions, the synaptic currents are blocked by c-bungarotoxin.7. Somatic motoneurones and vagal neurones induce junctional folds in the muscle membrane where they contact it to form an ectopic end-plate. Staining the