Experimental posterior uveitis was induced by the inoculation of retinal S-antigen into black hooded Lister rats. The time course of the disease was monitored by electroretinography (ERG), and the ERG changes were correlated with clinical signs and underlying pathological damage. The ERG became supernormal in the third week after inoculation, with some loss of temporal resolution (lowered ERG flicker fusion frequency), though the disease was not clinically manifest at this time and histological examination was normal. The ERG became subnormal after 21 days as clinical signs of disease began to appear. This subnormality was associated with focal photoreceptor necrosis, the degree of destruction being proportional to the reduction in the ERG. The ERG recovered with resolution of disease in most cases, though return to control values was uncommon. We suggest that the initial supernormal ERG reflects an underlying biochemical change mediated by the action of anti retinal S-antigen antibodies.