To dissect the possible role of the transforming growth factor-beta (TGF-beta) family in the regulation of skeletal muscle growth during the early postnatal period, the protein abundances of the TGF-beta family and their correlation with protein synthesis were determined in skeletal muscle of neonatal rats. To obtain direct evidence of the role of these growth factors in the regulation of protein synthesis, the TGF-beta inhibitor, follistatin, was infused into 10-d-old rats for 11 d and protein synthesis and phosphorylation of S6 kinase 1 (S6K1) and ribosomal protein (rpS6) were measured. TGF-beta2 abundance and protein synthesis in muscle decreased with development and were positively correlated. The abundances of bone morphogenetic protein 2 (BMP-2), BMP-7, and myostatin increased with development and were negatively correlated with protein synthesis. The abundances of BMP-2 and BMP-7 were positively correlated with BMP receptor IA (BMP-RIA) abundance. Activin A abundance was positively correlated with follistatin abundance and activin receptor IIB (Act-RIIB) abundance. Infusion of follistatin increased muscle protein synthesis and S6K1 and rpS6 phosphorylation. The results provide indirect and direct evidence of TGF-beta family involvement in the regulation of muscle protein synthesis during the neonatal period.