Studies have been carried out to characterize radioactive incorporation rates and steady-state levels of hyaluronic acid (HA) and chondroitin sulfate (CS) as well as hyaluronidase activity in the hind limbs of normal and brachpod mouse embryos between 11-14 days of gestation. The results of the analysis show that changes in the synthetic and degradative rates of HA and CS occur at about the 12.5-day stage in normal hind limbs. These changes include an increased rate of CS synthesis, a decreased rate of HA synthesis, and a correspondingly sharp, transitory rise in hyaluronidase activity. Similar changes also occur in brachypod hind limbs but appear to be delayed in onset by approximatly one-half day. In addition, the mutant hind limb exhibits a slower loss of HA at a time when turnover would be expected to be on the increase. These changes are concomitant with cell surface alterations and abnormal mesenchymal condensation formation which have been previously shown to occur in this mutant.