The activity of hyaluronate lyase was studied in the serum and testis of the rat. Depolymerisation of hyaluronic acid (prepared from human umbilical cord) was determined by loss of viscosity after incubation with serum or testis homogenate. A loss of this enzyme activity with increasing age was found in the testis but not in the serum. In old animals, treatment with Folcysteine resulted in a return of enzyme activity towards the values found in young animals.