The viscoelastic properties of the vitreous body from bovine and porcine eyes were determined by microrheometry. Each vitreous sample was sectioned into anterior, central and posterior segments. The rheological properties of each species and region were compared with each other and with the viscoelastic properties of human vitreous reported earlier. The results showed significant variations among species, as well as between regions in all species. All regions of human vitreous have significantly different rheological properties from those of the cow, and from the anterior and posterior regions of the pig; however, the viscoelastic behavior of the central porcine region closely resembles that of the human. In comparing the three species, the major differences found are in those parameters characterizing "viscous" or dissipative properties; values of "elastic" or energy storage properties were similar.