Distinct classes of acetylcholine receptor channels are formed when Xenopus oocytes are injected with combinations of the bovine alpha-, beta-, gamma- and delta- or the alpha-, beta-, gamma- and epsilon-subunit-specific messenger RNAs. The conductance and gating properties of the two classes of channels, in conjunction with the developmental changes in the muscular contents of the mRNAs, suggest that replacement of the gamma-subunit by the epsilon-subunit is responsible for the functional alteration of the receptor during muscle development.