Tritiated water and 14C-urea were introduced into one knee joint of twenty-one rabbits, and at intervals ranging from one to 360 minutes, articular cartilage from both knees and serum were assayed for retained radioactivity to provide a water or urea clearance for cartilage. By fifteen minutes less than 6 per cent of either isotope remained in the cartilage, and by thirty minutes equilibration with serum had occurred. In vitro incubation of slices of articular cartilage in 3H2O to assess the amount of retained water after vacuum dessication, or after oven drying for four to eighteen hours, showed that about 6 per cent of the retained water remained after mild dehydration (vacuum dessication) and that the amount diminished with harsher treatment but not to zero. The findings indicate that most of the water in the cartilage was mechanically trapped in a complex gel of matrix collagen and prdeoglycan and was freely exchangeable with both the synovial fluid and the blood stream.