SUMMARY1. The development of miniature end-plate currents (m.e.p.c.s) was studied in the superior oblique and interhyoideus muscles of Xenopus laevis. An analysis of m.e.p.c. decays shows that each muscle possesses its own characteristic programme ofend-plate current development.2. In the superior oblique, the exponential decay constants of m.e.p.c.s were initially about 3 ms; they declined within half a day to 1 ms and remained at that value for six weeks. They then gradually became longer, reaching a mean value of 1-7 ms at late metamorphosis.3. In the interhyoideus, m.e.p.c. decay constants were initially about 6 ms. They declined in less than one day to a mean value of 2-6 ms and remained there for the following seven weeks. Upon completion of metamorphosis, the decay constants underwent a further decrease to about 1 ms.4. In both muscles, the changes in m.e.p.c. decays were correlated with developmental changes in muscle contraction speeds, as measured by maximum twitch frequencies.5. The above changes inend-plate currents in the superior oblique andinterhyoideus muscles are discussed in terms of the development of acetylcholine receptor channel gating and acetylcholinesterase activity.