The relative spectral sensitivity of the photopic component of the cat's electroretinogram was measured in moderate light adaptation between 418–673 mμ by using flickering stimuli of constant rate. At slow rates of stimulation (10 to 20 flashes per second), the electrical response is dominated, except for longer wave-lengths, by scotopic components. Thus, photopic sensitivities can only be measured from about 552 to 673 mμ. With faster rates of flickering (above 30 per seconds), however, scotopic components appear to fuse, and photopic sensitivities for the whole visible spectrum can be obtained. The sensitivity distribution of the photopic component was found very similar to Dartnalls nomogram photopigment 556 mμ, but it is slightly narrower on both sides of the maximum. This effect is believed to be due to the reflectivity of the tapetum. Comparing the size of response on illumination of the tapetum and the tapetum-free area of the lensless cat's eye, one finds, that the intensity required for equal response is 0,45 to 0,75 logarithmic units lower for the tapetum than for the tapetum-free area.