Abstract. The level of transcripts encoding the skeletal muscle acetylcholine receptor (AChR) was determined during embryonic development in Xenopus laevis, cDNAs encoding the alpha, gamma, and delta subunits of the Xenopus AChR were isolated from Xenopus embryo cDNA libraries using Torpedo AChR cDNAs as probes. The Xenopus AChR cDNAs have >60% amino acid sequence homology to their Torpedo homologues and hybridize to transcripts that are restricted to the somites of developing embryos. Northern blot analysis demonstrates that a 2.3-kb transcript hybridizes to the alpha subunit cDNA, a 2.4-kb transcript hybridizes to the gamma subunit cDNA, and that two transcripts, of 1.9 and 2.5 kb, hybridize to the delta subunit cDNA. RNase protection assays demonstrate that transcripts encoding alpha, gamma, and delta subunits are coordinately expressed at late gastrula and that the amount of each transcript increases in parallel with muscle-specific actin mRNA during the ensuing 12 h. After the onset of muscle activity the level of actin mRNA per somite remains relatively constant, whereas the level of alpha subunit and delta subunit transcripts decrease fourfold per somite and the level of gamma subunit transcript decreases >50-fold per somite. The decrease in amount of AChR transcripts per somite, however, occurs when embryos are paralyzed with local anaesthetic during their development, These results demonstrate that AChR transcripts in Xenopus are initially expressed coordinately, but that gamma subunit transcript levels are regulated differently than alpha and delta at later stages.Moreover, these results demonstrate that AChR transcript levels in Xenopus myotomal muscle cells are not responsive to electrical activity and suggest that AChR transcript levels are influenced by other regulatory controls.T I~I~ skeletal muscle acetylcholine receptor (AChR) ~ is the most thoroughly studied and well understood neurotransmitter receptor. The AChR is a complex composed of four different, but structurally related, subunits (alpha2, beta, gamma, delta) that form a ligand-gated channel (Karlin, 1980). During synapse formation the cell surface distribution, metabolism, and channel kinetics of AChRs are altered and these alterations have important functional consequences for synaptic transmission (Fambrough, 1979). Nevertheless, the steps and mechanisms that control these biochemical and physiological changes are poorly understood,To understand more about how AChR expression is controlled during muscle differentiation, we investigated whether transcripts encoding different subunits of the AChR are expressed coordinately during early embryonic development in vivo. We studied AChR expression in Xenopus laevis so that initial expression of AChR transcripts could be determined and so that results from these studies could be placed in context with the wealth of information regarding muscle differentiation and synapse formation in Xenopus (Anderson and 1. Abbreviation used in this paper: AChR, acetylcholine receptor. Kullberg et al., 197...