Fibrocartilage is found in tendon at sites where the tissue is subjected to transverse compressive loadingin vivo.A significant characteristic of the tissue transition from tendon to fibrocartilage in bovine deep flexor tendon is increased gene expression, synthesis, and accumulation of both a large proteoglycan, aggrecan, and a small proteoglycan, biglycan. In order to investigate the cellular events involved in this response, segments of fetal bovine deep flexor tendon were subjectedin vitroto cyclic compressive load for 72 h. Following loading, the level of aggrecan mRNA in cells from loaded tissue was increased 200–450% compared to matched nonloaded tissue segments, as determined by slot-blot analysis. The level of biglycan mRNA increased 100%, and the level of versican mRNA increased 130% in the loaded tissue. The level of decorin mRNA remained virtually unchanged, while expression of α1(I) collagen increased only 40%. When tissue segments were cultured in the presence of transforming growth factor (TGF)-β1 (1 ng/ml), the synthesis and expression of mRNA for both aggrecan and biglycan increased, whereas decorin expression was not affected. Similarity in both the direction and the pattern of the cellular response to mechanical load and TGF-β suggested a causal relationship. Both loading of tendon segments and TGF-β treatment increased expression of mRNA for TGF-β by ∼40% compared to control tissue. In addition, the amount of newly synthesized TGF-β immunoprecipitated from extracts of loaded tissue was several-fold greater than that from nonloaded tissue. The experiments of this study support a hypothesis suggesting that one aspect of the response of cells in fetal tendon to compressive load is increased TGF-β synthesis which, in turn, stimulates synthesis of extracellular matrix proteoglycans and leads toward fibrocartilage formation.