To investigate pattern electroretinogram changes in treated ocular hypertension, we evaluated pattern electroretinogram recordings of 48 hypertensive eyes following an 8-month timolol maleate therapy. During treatment, 27 of 48 eyes had normalized intraocular pressures (15-18 mm Hg), while 21 retained elevated values (21-25 mm Hg). Twenty-eight eyes with untreated hypertension (22-25 mm Hg) lasting at least 8 months, as well as 32 untreated, normotensive eyes served as controls. When compared to untreated normotensive controls, timolol-treated eyes with either elevated or normalized intraocular pressures showed reductions in the mean electroretinographic amplitudes. However, these amplitude reductions were substantially greater in treated eyes with elevated pressures as compared to those with normalized ones. Untreated hypertensive controls showed pattern electroretinogram reductions, with respect to normal values, that were comparable to those of treated hypertensive eyes, but larger than those of treated normotensive ones. These results indicate that, in treated ocular hypertension, pattern electroretinogram losses tend to be associated with moderately increased intraocular pressures in the range of 21-25 mm Hg. Electroretinographic abnormalities may be, at least in part, prevented only by lowering intraocular pressure into a normal range.