Synovial perfusion was quantified in milliliters per minute per knee by two quite different clearance methods based on (1) counting tritiated water in serial aspirates of intraarticular saline, and (2) external counting of joints injected with free radioiodide. In each case, the serial counting data determine a rate constant that is multiplied by a distribution volume to provide the clearance in flow terms of milliliters per minute. This report updates and summarizes these data and compares the two methods to each other and to alternative assessments of synovial blood flow. Available methods such as laser Doppler flowmetry (with data output measured in volts) and solute clearance constant determinations (in min-1) are useful for selected purposes but cannot be used to quantify the articular flux (in milligrams per minute) of any solute. Radiolabeled microspheres provide data (in milliliters per minute per g of tissue) but are unsuitable for human use. The two clearance methods provide comparable results, but the free iodide technique seems most suitable for physiologic investigations. The latter potentially includes critical evaluations of synovial blood flow in relation to issues such as palpable warmth, visible erythema, articular ischemia, the permeability of synovial vessels, the genesis of effusions, the delivery and removal of therapeutic agents, and the concentration of every synovial fluid solute from micronutrients through cytokines, plasma proteins, and molecular markers of cartilagenous injury.