The biosynthesis of fibronectin and laminin was studied in satellite cells cultured from adult rat muscles before (day 4) and after fusion and formation of myotubes (day 14) using (35S) methionine as a tracer. The kinetics of incorporation into immunoprecipitable fibronectin and laminin were recorded at intervals from 1.5 to 24 hours of incubation with the tracer from the cells, the pericellular matrix and the culture medium. The rate of synthesis of fibronectin and laminin expressed as dpm/microgram DNA were constant from the mononucleated cell to the myotube state. Both glycoproteins were detected in the cells and in the pericellular matrix. When the results were expressed as the percentage of incorporation into total protein, major changes could be observed in the early phase of the kinetic studies in the cells and the pericellular matrix. Both showed an increase from the mononucleated myoblast to myotube, suggesting that an increasing fraction of total protein biosynthesis is directed towards these two extracellular matrix glycoproteins. At the same time, there was a decrease in the secretion into the medium of freshly synthesized radiolabeled fibronectin and laminin. Our results confirm the synthesis of varying ratios of both extracellular matrix macromolecules by undifferentiated mononucleated myogenic cells as well as myotubes.