To investigate the metabolism of proteoglycans in young growing rats, calvaria, incisors, femoral diaphysis and metaphysis were labelled in vitro for 0.5-72 h with [35S]sulphate. At each time point the specific radioactivity, expressed as c.p.m. of [35S]sulphate/μg of uronic acid, of papain-resistant macromolecules in each tissues was determined. The identity of the glycosaminoglycans was established by the use of specific enzymic and chemical methods of degradation. Incorporation of the label into each tissue was maximal at 12 h; it then declined to 50-75% of that value by 72 h. Chondroitin sulphate was the predominant glycosaminoglycan in each tissue, representing 80-96% of the total; heparan sulphate comprised 2-14% of the total; in general, radioactive material sensitive to keratanase comprised less than 1% of the total. The relative amount of labelled chondroitin sulphate increased, whereas that of heparan sulphate decreased, with increasing time of incorporation. These data show that 25-50% of the newly synthesized glycosaminoglycans are lost from mineralizing tissues, during the time in which the newly secreted organic matrix becomes mineralized.