The purpose of this study was to determine if age-related muscle atrophy is associated with an increased rate of protein degradation in extensor digitorum longus (EDL) muscles from young (YG; 2–4 months), middle-aged (MA; 12–17 months), and aged (AG; 22–24 months) B6C3F1 mice. EDL muscles from AG mice weighed less than EDL muscles from MA mice (p = .01). EDL muscles from MA mice weighed more than EDL muscles from YG mice (p = .02). The rate of protein degradation, as assessed by tyrosine release during in vitro incubations, was higher in EDL muscles from AG mice than it was in those from MA mice (p = .03). The rate of protein degradation was higher in EDL muscles from YG mice than it was in those from MA mice (p = .04). An inverse relationship existed between muscle mass and protein degradation (r = −.67; p = .0001). We conclude that skeletal muscle protein degradation rates decrease with maturation and increase with advancing age.