Organ culture methods were used to study the effect of parathyroid hormone (PTH) on incorporation of 6-3{H-glucosamine into macromolecules during bone resorption. Glucosamine- labeled macromolecules which remained nondialyzable after papain digestion were separated by chromatography on DEAE cellulose. In bones cultured with PTH, the fraction containing hyaluronate showed a dose-dependent increase in labeling which was closely correlated to the dose-dependent increase in bone resorption as measured by release of 45Ca into the medium. The labeling of glycoproteins eluting before hyaluronate and of sulfated polysaccharides eluting after PTH was not significantly increased by PTH treatment. The increased labeling of hyaluronate was associated with increased content of glucosamine and glucuronate residues in the hyaluronate fraction, without large changes in specific activity. The presence of PTH was not required during the labeling period to observe the effect. The increase in hyaluronate labeling and content was attributed to increased synthesis and not decreased turnover because sequential analyses of pulse-labeled bones showed no effect of PTH treatment on the fractional degradation rates of hyaluronate. The increase in hyaluronate labeling after PTH was significant in the first 4 hr, and peaked at 24 hr. The changes in 45Ca release with PTH showed a parallel time course but a delayed onset and peak. These findings indicate that hyaluronate synthesis is increased when bone resorption is stimulated by PTH, and suggest that this increased synthesis may be essential for the activity of bone resorbing cells. (Endocrinology94: 737, 1974)