We study the elimination of two viscoelastic substances from the vitreous cavity: the Hydroxypropyl Methylcellulose and the Sodium Hyaluronate that are potentially vitreous substitutes. We performed gas-vitrectomy in 116 eyes of 58 rabbits. Three days after surgery we performed gas-viscoelastic substance exchange. We analyzed the concentration of Hydroxypropyl Methylcellulose and Sodium Hyaluronate with diphenylamine reaction at different periods: zero, 1 week, 2 weeks, 1 month till 6 months. In both cases the elimination is fast so that 1 week after the intravitreal injection remains only 60.1% of the Hydroxypropyl Methylcellulose and 100% of Sodium Hyaluronate, and two weeks after 38.5% of Hydroxypropyl Methylcellulose and 62.5% of Sodium Hyaluronate. We conclude that both substances have a short half life-time specially in the case of the Hydroxypropyl Methylcellulose.