To determine the precise mechanism by which contact tendon healing occurs at the cellular level, the production of pro α(I) collagen messenger RNA (mRNA) produced by fibroblasts of healing intrasynovial flexor tendons was determined by an in situ hybridization technique. The repair site and the proximal and distal tendon stumps of repaired tendons treated with early controlled passive mobilization were fixed and buffered in formalin, 3, 7, 10, and 17 days after repair. A complimentary DNA (cDNA) probe corresponding to α(I) procollagen mRNA was labeled with [32P]d-CTP. After hybridization, autoradiography, and staining of the sections, the level of procollagen mRNA was assessed by microscopic examination. Rising levels of procollagen mRNA, indicating progressively increasing levels of synthetic collagen activity, were detected in the healing tendons through 10 days. A moderate decrease in procollagen mRNA was seen at 17 days. Genetic expression for procollagen mRNA was localized specifically to the epitenon cells on the tendon surface overlying the repair site and to cells in the gap between the tendon stumps. No detectable expression was noted in endotenon fibroblasts. The finding of high levels of expression for procollagen type I mRNA in the surface layer of healing tendons demonstrates that cells intrinsic to tendon epitenon contribute the greatest quantity of native tendon collagen to the repair site during these important early intervals after tendon suture.