Twenty-four patients with proliferative or preproliferative diabetic retinopathy underwent extensive argon laser photocoagulation in one eye. Detailed central and peripheral visual-field examinations and electroretinography were performed before treatment, and at intervals after treatment. Visual field changes were as follows: (1) eight patients had only mild to moderate constriction of all isopters, (2) 11 had discrete scotomata in addition, (3) two had prominent nerve-fiber-bundle defects, (4) three had severe constriction of all isopters, save (in two) that to the largest peripheral test object. Electroretinographic b-wave amplitudes were reduced an average of 40% to white test flashes. Blue test flashes showed an even larger reduction of the rod response. This suggests the receptors in approximately 40% of the retinal area are destroyed by such extensive photocoagulation, covering predominantly that part of the retina that has the highest concentration of rods.