To investigate changes in muscle metabolism during lactation, serial biopsy of the triceps brachii was conducted in first-parity sows subjected to three degrees of selective protein mobilization through restriction of dietary protein intake (see Clowes EJ, Aherne FX, Foxcroft GR, and Baracos VE. J Anim Sci 81: 753-764, 2003). Muscle biopsies were taken 7 days before parturition and at 12 and 23 days of lactation. The following changes occurred after parturition, were progressive, and were significantly magnified in animals under the greatest degree of dietary protein restriction and hence of protein mobilization. Decreased RNA-to-DNA ratio (capacity for protein synthesis) was observed. The presence of increased expression of several elements of the ubiquitin proteasome proteolytic pathway suggested a robust catabolic response. However, as lactation progressed, and especially under conditions of increased dietary protein restriction, protein mobilization increased, muscle RNA-to-DNA ratio fell further, protease gene expression continued to rise, tissue free glutamine levels rose dramatically, and essential amino acid levels, especially branched-chain amino acids and threonine, fell to below prepartum levels.