Foveal cone electroretinograms were elicited with a hand-held stimulator-ophthalmoscope from 16 patients with retinitis pigmentosa and 17 patients with juvenile macular degeneration. Among 11 patients with retinitis pigmentosa and visual acuities of 6/9 (20/30) or better, eight had foveal cone ERGs that were normal in amplitude and b-wave implicit time; all five patients tested with visual acuities of 6/12 (20/40) to 6/18 (20/60) had foveal cone ERGs that were reduced in amplitude and normal in implicit time. All patients with juvenile macular degeneration and visual acuities of 6/15 (20/50) or worse showed subnormal amplitudes with normal or delayed implicit times, or responses indistinguishable from noise. Amplitudes tended to be smaller and delays greater among patients with juvenile macular degeneration and visual acuities of 6/60 (20/200) or worse.