Electroretinographic (ERG) responses were measured in diabetic patients before, during, and after panretinal photocoagulation treatment with argon laser. The laser applications reduced considerably the amplitudes of the a and b waves of the ERG. Moreover, the relationship between the amplitude of the b wave and that of the a wave was severely affected, resulting in ERG responses of abnormal pattern. The b waves were smaller than expected from the a waves. These findings indicated that the photocoagulation treatment not only destroyed the retinal areas directly illuminated by the laser beam, but also affected the functional integrity of adjacent areas. These additional effects resulted in subnormal signal transmission from the photoreceptors to the proximal retina.