Microspheres of 3 different sizes were infused separately into the eyes of dogs with normotensive and hypertensive intraocular pressures. Latex spheres (0.5 or 1.0 micron) or 3.0 micron plastic spheres were added to Ringer's solution with 6% gelatin. Initially, this mixture was injected into the anterior chamber of dogs with intraocular pressures of 20, 50, or 75 mm of Hg. After 10, 20, 30, 60, or 90 minutes had elapsed, the dogs were euthanatized and the gelatin was hardened. Tissues were subsequently studied by light and transmission electron microscopies. Phagocytosis of the 0.5 and 1.0 micron spheres by trabecular cells was first detected within 10 minutes and within 20 minutes for the 3.0 microns spheres. Migration of cells in the corneoscleral trabecular meshwork was observed after 30 minutes. Phagocytosis was less active at hypertensive pressures and had larger sphere sizes. Microspheres in the uveal trabecular meshwork were ingested mostly by macrophages and polymorphonuclear leukocytes.