During the development of mammalian muscle the ~-subunit of the nicotinic acetylcholine receptor (AChR) is replaced by the e-subunit to produce well-defined alterations in the conductance and gating of the channel. To gain a better unterstanding of the functional role of the V-and e-subunits, we have studied the properties of an AChR channel lacking these subunits. The AChR expressed in Xenopus oocytes injected with the bovine a-, /3-and dsubunit-specific mRNAs (referred to as a/3d-AChR) is unusual in that its channel opens spontaneously at a high frequency in the absence of agonist. From a comparison of the a/36-AChR with complete receptors containing either the ~-or e-subunit, we conclude that the y-and e-subunits influence most channel properties, including agonist binding, and are especially important for stabilizing the closed state of the unliganded receptor channel. The afid-AChR can form when a complete set of four subunit-specific mRNAs is injected. The ease with which it is assembled raises the possibility that the a/3d-AChR contributes to some of the variations in receptor properties that occur during development.