We have examined normal and denervated mouse skeletal muscle for histological evidence of changes in RNA concentration using acridine orange/nucleic acid fluorescence. In denervated muscle, cytoplasmic RNA concentration increases dramatically. The earliest increase is in the subsarcolemmal region, 7 days after denervation. By 28 days after denervation, the increase in RNA is distributed throughout the cytoplasm of the atrophic cells, non-atrophic cells show little or no increase. Prior to the increase in cytoplasmic RNA concentration, many myocyte nuclei exhibit enlarged nucleoli, indicative of active ribosomal RNA synthesis. Such active nucleoli are present throughout the 42 day study period. We conclude that both new RNA synthesis, and preferential accumulation of cytoplasmic RNA are prominent events in denervation atrophy. There is an ordered progression from nucleolar synthesis to subsarcolemmal accumulation to diffuse accumulation in atrophying cells. These observations complement biochemical findings of others which indicate an increase in both RNA and protein synthesis after denervation.