Orthograde and retrograde axoplasmic transport have been studied in the optic nerve heads of 37 Macaca fascicularis eyes with normal or elevated intraocular pressure (IOP) produced by cannulation of the anterior chamber. Orthograde transport was labeled by 3H-amino acids injected intravitreally and incorporated into retinal ganglion cell proteins. Retrograde transport was studied in the same eyes by injecting horseradish peroxidase (HRP) into one or both optic tracts and dorsal lateral geniculate nuclei (dLGN). Both tracers accumulated in the lamina scleralis (LS) of eyes maintained at pressures of 25 to 150 mm. Hg for 12 to 28 hours (pressure in normal controls = 10 to 14 mm. Hg) but the HRP technique was markedly more sensitive. The degree of retrograde transport obstruction in the LS appeared to be directly proportional to both the height and the duration of elevated IOP. In one experiment, the blockades of orthograde and retrograde transport induced at 50 mm. Hg were demonstrated to be reversible. Serial reconstructions of radioautographs and peroxidase-reacted sections of the optic nerve heads demonstrated that the orthograde and retrograde transport obstructions were coincidental anatomically by light microscopy in the LS and occurred most prominently in the temporal quadrants of the nerve head. These transport obstructions occurred at moderate elevations of IOP (25 TO 50 mm. Hg) despite (1) elevated arterial PO2 levels during inhalation of 100 percent oxygen and (2) intact nerve head capillary circulation, as demonstrated by perfusion with nucleated avian erythrocytes.