Basic mechanisms of the rat ERG were reinvestigated with contemporary methods of recording and photic stimulation via an optic fiber system connected with a contact lens. Flash stimulation and background illuminance were performed with photometrically defined light stimuli. ERGs recorded in darkness, from dark adapted rats, were similar with those observed in earlier work in which light flashes of much longer duration had been used. Flash stimulation carried out under stepwise increased background illuminance gave new information on the characteristics of oscillatory potentials of the ERG. In general the present observations agreed with the notion that the rods of the rat retina are as sensitive as in man, whereas the cones are functionally less efficient with respect to light sensitivity and temporal resolution. Differences in function of the cones from rat to man have to be kept in mind when using the rat ERG as a model for risk assessment in safety studies.