Myostatin is hypothesized to regulate skeletal muscle mass and to be a potential target for therapeutic intervention in sarcopenia. To clarify whether myostatin is invariably associated with sarcopenia, this study examined the levels of expression of myostatin mRNA and protein in Sprague Dawley rats during aging- and denervation-induced sarcopenia. The level of myostatin mRNA in the gastrocnemius decreased progressively with age being 9, 34 and 56% lower at 6, 12 and 27 months, respectively, compared with mRNA levels at 1.5 months. In contrast, two low molecular mass isoforms of myostatin protein identified by Western blotting increased progressively with age. With denervation, myostatin mRNA was 31% higher on day 1 but by 14 days after sciatic neurectomy when the muscle had atrophied 50%, myostatin expression decreased 34% relative to the sham operated limb. Western analysis of the denervated gastrocnemius showed that myostatin protein levels varied in parallel with mRNA. These disparate patterns of expression of myostatin during age- and denervation-induced atrophy suggest that the regulation of myostatin is complex and variable depending on whether the atrophy is slowly or rapidly progressive.