A pattern electroretinogram (PERG) was recorded in patients in the initial stages of visual field defects resulting from open-angle glaucoma, in patients with ocular hypertension (OHT) and in age-matched controls. In both groups the visual acuity was above 0.8. Counterphasing checkerboard patterns were used as visual stimuli with a check size of 0.8 degrees and a range of reversal rates from 6 to 36 reversals/s. The most significant reductions of amplitudes due to glaucoma and OHT were observed at rates of 18 and 24 reversals/s (glaucoma: P less than 0.005 at 18 and 24 reversals/s, OHT: P less than 0.0005 at 18 reversals/s and P less than 0.0025 at 24 reversals/s). At reversal rates of 6 and 36 reversals/s the PERG was either not reduced (glaucoma and OHT: P greater than 0.01 at 36 reversals/s, OHT: P greater than 0.01 at 6 reversals/s) or reduced only marginally (glaucoma: P less than 0.01 at 6 reversals/s). When one takes the 95% confidence limits of normals into consideration, it is obvious that patients with glaucoma and patients with OHT have pathologic PERGs more often at 18 reversals/s than at other temporal frequencies.