Isolated single fascicles from tail tendons of young rats were freed of epitenon cells and cultured in vitro for up to 7 days. The tissue remained viable, as judged by the structural integrity of cell organelles and the ability to synthesize DNA and glycosaminoglycans (GAG). The rate of DNA synthesis peaked after 2 days in culture and decreased slowly thereafter. Concomitantly, an increase in cell number was noted at the periphery of the fascicle. GAG production also increased during culture, sulphated GAG being increased proportionately more than hyaluronic acid. Dermatan sulphate was the predominant sulphated GAG in freshly isolated fascicles, but in cultured tissue, the newly synthesized sulphated GAG was more sensitive to degradation by chondroitinase AC and had an increased electrophoretic mobility. fine structural changes were observed in cultured tissues such as the retraction of cell processes. rounding up of cell bodies and the appearance of gaps between collagen fibrils. Cultured tenocytes also frequently contained apparently phagocytized collagen fibrils which were not seen in freshly isolated fascicles, and this appearance was suggestive of collagen degradation occurring in vitro, although no change in the total hydroxyproline content was noted. The data show that when individual fascicles are cultured in vitro they undergo a process of matrix remodelling which has features in common with events occurring in vivo when tendons have been surgically manipulated.