Rod ERGs were obtained over a broad range of stimulus intensities from young patients with retinitis pigmentosa and progressive cone-rod degeneration. Intensity-response functions suggest that children with retinitis pigmentosa have reductions in the number of functioning rods and that surviving rods generating the ERG have reduced sensitivity. Children with cone-rod degeneration also have reduced numbers of rods, but the remaining rods have normal sensitivity. These findings suggest that the underlying mechanisms of rod degeneration differ in these two forms of disease and demonstrate that insights into the pathogenesis of hereditary retinal degeneration can be obtained by testing children. The ability to distinguish between retinitis pigmentosa and cone-rod degeneration in children is also important because of differences in the rates of progression between the two classes of progressive retinal degeneration.