Changes in the electroretinogram (ERG) are studied in patients with large chorioretinal scars and certain hereditary retinal diseases. Amplitude and implicit time of the ERG responses are shown to behave independently of one another in these conditions. Chorioretinal destruction produces a reduction in amplitude but no change in implicit time. Hereditary disease leads to reduction in amplitude which can be accompanied by changes in implicit time. Some degenerations produce changes in implicit time before changes in amplitude. The delays in implicit time can be best explained by an abnormality of the rod and/or cone receptor system involving the entire retina. Alterations of implicit time due to the intensity and area of light stimulation and the state of retinal adaptation are considered.