Recent evidence suggests that retinal hypoxia and ischemia affect the standing potential of the eye and the activity of the photoreceptors. To test whether chronically elevated intraocular pressure would produce similar effects, we measured electro-oculograms in two groups of patients: ocular hypertensive patients and patients with primary open-angle glaucoma. There were significant differences among the average electro-oculogram ratios of these groups compared to age-similar controls. The control observers had an average light-peak/dark-trough ratio of 2.86, the ocular hypertensive patients had an average ratio of 2.44, and the patients with primary open-angle glaucoma had an average ratio of 2.07. This indicates that long-term elevations in intraocular pressure can decrease the light peak of the electro-oculogram, even in patients with no other evidence of glaucomatous damage. This deficit may have its origins in the sensitivity of the outer retina to choroidal ischemia.