1. 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 tau ACh = 1.8 +/- 0.1 msec (S.E.) at -70 to -90 mV and 15-18 degrees C. Carbachol- and suberyldicholine-induced channels have average open times tau Carb = 1.0 +/- 0.1 msec (-80 mV, 6-8 degrees C) and tau Sub = 3.3 +/- 0.1 msec (-80 mV, 16-18 degrees C). 3. The conductances gamma of single channels induced by ACh, carbachol and suberyldicholine are: gamma ACh = 21.0 +/- 1.1 pS (+/- S.E.), gamma Carb = 14.6 +/- 1.2 pS and gamma Sub = 22.1 +/- 1.2 pS. 4. The mean open times tau 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 tau = 1.6 +/- 0.2 msec (S.E.) at -70 to -80 mV and 14-15 degrees C. At denervated motor end-plates reinnervated by vagal neurones, the mean channel open time is tau = 1.5 +/- 0.1 msec at -80 mV and 14-18 degrees C. 6. At all vagus junctions, the synaptic currents are blocked by alpha-bungarotoxin. 7. Somatic motoneurons and vagal neurones induce junctional folds in the muscle membrane where they contact it to form an ectopic end-plate. Staining the ACh receptors of the motor end-plates with horseradish peroxidase-alpha-bungarotoxin conjugate shows that the receptors are restricted to the junctional region of the muscle fibre membrane.