The muscular content of the mRNAs encoding the five subunits of the nicotonic acetylcholine receptor was measured during postnatal development in the rat. Subunit specific mRNAs show differential regulation. The levels of the alpha-, gamma- and delta-subunit specific mRNAs decrease steadily after birth, while the beta- and epsilon-subunit mRNAs increase transiently and then decrease. The adult pattern of subunit specific mRNA levels is reached at 4-6 weeks postnatally. The content of gamma- and epsilon-subunit mRNA changes in a reciprocal fashion during the first 2 postnatal weeks, supporting the view that differential regulation of gamma- and epsilon-subunit mRNA during development is one mechanism mediating the appearance of the adult, epsilon-subunit containing, subtype of end-plate channel. Denervation of neonatal muscle increases the levels of all subunit-specific mRNAs during further development. It prevents the postnatal decrease in gamma-subunit mRNA and enhances the initial increase in epsilon-subunit mRNA. This makes it appear that the epsilon-subunit gene is less sensitive to regulation by the nerve in the postnatal period than the gamma-subunit gene.