Silent substitution and selective adaptation techniques were used to obtain full field S-cone and L + M-cone electroretinograms from 18 patients with ocular hypertension (OHT), 9 with normotensive glaucoma (NTG), 18 with early primary open angle glaucoma (POAG) and 19 normal controls. Pattern electroretinograms were also recorded, using a reduced check size to increase the contribution of retinal ganglion cells. In the OHT and POAG groups, statistically significant reductions (P = 0.05-0.001) were observed in the amplitudes, most notably in the late negative waves of all three types of ERG compared to the controls. These are thought to reflect ganglion cell activity. The results imply a diffusely distributed loss of activity (20-35%) affecting many retinal pathways to a similar extent in OHT and early POAG, with an additional amount (<5%) in POAG corresponding approximately to the loss associated with local field defects. The electrophysiology indicated that virtually all cases of untreated OHT have greater retinal dysfunction than the least affected cases of POAG. The NTG group showed a different pattern of loss in that the PERG was markedly affected but the S-cone ERG was not significantly reduced.