The mRNA coding for the alpha-subunit of the acetylcholine receptor was studied in mouse leg and rat diaphragm muscle. We find that denervation of rat diaphragm results in a 7-fold increase in mRNA coding for the alpha-subunit, whereas denervation of mouse leg muscle results in approximately a 50-fold increase in alpha-subunit-specific mRNA. The relationship of the mRNAs purified from innervated and denervated muscle was investigated by SI nuclease mapping. Two mRNA species were found in both innervated and denervated muscle which differ in their 3'-untranslated region. The levels of both these mRNA species increase upon denervation of mouse leg muscle.