The role of synovial fluid in providing nutrition to rabbit knee ligaments and menisci was evaluated by intraarticular injection of a labeled collagen precursor, tritiated proline. Incorporation of this substrate as tritiated hydroxyproline was measured in collateral and cruciate ligaments and menisci. The injectate volume (0.35 ml) did not appreciably change the overall joint pressure as measured by a wick catheter; therefore, no alteration of synovial membrane diffusion characteristics resulted. The concentration of the injected proline (0.52 mg%) was well below that normally present in serum (2.65 mg%). Therefore, incorporation of this substrate was not driven by a concentration gradient and represented normal uptake of synovial fluid and physiological incorporation of label as measured by the presence of tritiated hydroxyproline. Autoradiography was performed on all ligaments and menisci, and demonstrated concentration of the isotope and its metabolite (tritiated proline and tritiated hydroxyproline, respectively) in and around fibroblasts. This study indicates that rabbit knee ligaments and menisci can derive nutrition from a synovial fluid source.