Muscle adapts to exercise through changes in the abundance of specific contractile and metabolic proteins. Exercise training also increases muscle protein turnover, but it is not known how synthesis and degradation contribute to exercise-induced changes in the abundance of individual proteins. We used stable isotope (deuterium oxide) labelling in combination with chronic low-frequency stimulation (CLFS) in vivoto investigate the synthesis, abundance and degradation of individual proteins during exercise-induced muscle adaptation. Surgery was performed on 4 independent groups of rats (n = 3 in each) under aseptic conditions. Pre-operativeanalgesia 0.05 mg Buprenorphine /kg body mass was provided and anaesthesia was induced by4 % isoflurane-O2adjusted to 1-2 % isoflurane during surgery.Unilateral CLFS (10 Hz, 24 h/d) and deuterium oxide were administered for 0 d, 10 d, 20 d or 30 d. Rats were humanely killed and the extensor digitorium longus (EDL) was isolated from stimulated (Stim) and non-stimulated (Ctrl) legs. Proteomic analysis encompassed 30 myofibrillar and 47 soluble proteins. Protein fractional synthesis rate (FSR) was calculated from peptide mass spectrometry data. Absolute synthesis rates (ASR) were derived from FSR and abundance data and used to calculate protein degradation rates. Data are mean ± SD compared by within-animal paired t-test. CLFS tended (P = 0.145) to increase the synthesis of mixed myofibrillar proteins from Ctrl (FSR, 4.7 ± 0.3 %/d; ASR, 8.6 ± 0.3 pg/d) to Stim (FSR, 5 ± 0.1 %/d; ASR, 10.3 ± 1.6 pg/d). Whereas the synthesis of mixed soluble proteins increased (P = 0.001) from Ctrl (FSR, 5.2 ± 0.04%/d; ASR, 43.7 ± 0.1 pg/d) to Stim (FSR, 5.4 ± 0.4 %/d; ASR, 60 ± 0.7 pg/d). Protein turnover responses differed on a protein-by-protein basis. The abundances of glycogen phosphorylase(PYGB), beta-enolase(ENOB) and troponin T, fast (TNNT3) significantly (P<0.05) decreased (-11 ± 3-fold) after CLFS. The decrease in abundance of ENOB was partially accounted for by a lesser synthesis in Stim muscle. The decrease in PYGB abundance was entirely accounted for by the decrement in its synthesis rate, whereas the abundance of TNNT3 decreased without a detectable change in synthesis. Myosin regulatory light chain 2 (MLRS) was resolved as 2 separate proteoforms (a and b). MLRSa was uniquely phosphorylated at S20 and significantly (P< 0.05) decreased (62 %) in abundance, whereas MLRSb tended (P = 0.100) to increase (16 %) after 30 days of CLFS. There was no difference in synthesis rate between Stim and Ctrl muscle of either MLRS proteoform. Therefore, we attribute the decrease in MLRSa abundance to proteoform-specific degradation. In conclusion, muscle adaptation in response to CLFS is underpinned by protein-specific changes in degradation as well as synthesis.