An attempt was made to investigate the effects of hyaluronidase on the morphology of the baboon outflow system. The eyes of six adult baboons provided the material for this histological study. For each animal the IOP in both eyes was maintained at 18 mm Hg, and 150 IU of testicular hyaluronidase were introduced into one eye and a control solution into the other. It was observed by light microscopy that the outer meshwork was more distended in the hyaluronidase-treated outflow system than in the corresponding controls. Further, it was found from a quantitative analysis that the incidence of giant vacuoles in the endothelium of Schlemm's canal was greater in the experimental than the control eye of each animal. The difference between the vacuole counts in the experimental and control eyes was significant in five of the six animals. These preliminary findings provide morphological evidence which indicates that there is a hyaluronidase-sensitive barrier to aqueous outflow in the baboon drainage system.