The addition of proteinase inhibitors (1 mM phenylmethylsulfonyl fluoride, 10 mM N-ethylmaleimide, 0.25 mM benzamidine hydrochloride, 6.25 mM EDTA, 12.5 mM 6-aminohexanoic acid and 2 mM iodoacetic acid) to explant cultures of adult bovine articular cartilage inhibits proteoglycan synthesis as well as the loss of the macromolecule from the tissue. Those proteoglycans lost to the medium of explant cultures treated with proteinase inhibitors were either aggregates or monomers with functional hyaluronic acid-binding regions, whereas proteoglycans lost from metabolically active tissue also included a population of monomers that were unable to aggregate with hyaluronate. Analysis of the core protein from proteoglycans lost into the medium of inhibitor-treated cultures showed the same size distribution as the core proteins of proteoglycans present in the extracellular matrix of metabolically active cultures. The core proteins of proteoglycans appearing in the medium of metabolically active cultures showed that proteolytic cleavage of these macromolecules occurred as a result of their loss from the tissue. Explant cultures of articular cartilage maintained in medium with proteinase inhibitors were used to investigate the passive loss of proteoglycan from the tissue. The rate of passive loss of proteoglycan from the tissue was dependent on surface area, but no difference in the proportion of proteoglycan aggregate to monomer appearing in the medium was observed. Furthermore, proteoglycans were lost at the same rate from the articular and cut surfaces of cartilage. Proteoglycan aggregates and monomer were lost from articular cartilage over a period of time, which indicates that proteoglycans are free to move through the extracellular matrix of cartilage. The movement of proteoglycans out of the tissue was shown to be temperature dependent, but was different from the change of the viscosity of water with temperature, which indicates that the loss of proteoglycan was not solely due to diffusion. The activation energy for the loss of proteoglycans from articular cartilage was found to be similar to the binding energies for electrostatic and hydrogen bonds.