The inability of retinal pigment epithelium to phagocytose shed photoreceptor disks is a cause of retinal degeneration in the Royal College of Surgeons rat; retinal pigment epithelial phagocytosis and disk shedding are regulated by the diurnal rhythm of retinal melatonin level. The diurnal rhythms of the electroretinogram (particularly that of the retinal pigment epithelial potential, the electroretinographic c-wave) and retinal melatonin content were thus investigated in Royal College of Surgeons rats from postnatal day 17 to 24, the period preceding retinal degeneration. The amplitudes of both the b- and c-waves of the electroretinogram fell significantly during the peak time of rod disk shedding and rose after the time of expected light off in the control and dystrophic rats. While the b-wave rhythms did not differ between the two strains, diurnal changes in the c-wave were significantly less distinct in the dystrophic rats than in controls. This difference may reflect lack of phagocytosis in dystrophic rats. Furthermore, the ERG c-wave was significantly larger and prolonged, and the retinal melatonin content higher, in dystrophic rats of this age group than in controls. It appears that retinal melatonin metabolism may play an important role in the maintenance of retinal pigment epithelial and photoreceptor function.