The spectral sensitivity of the pigeon Electroretinogram was determined at different times during the 24-hour cycle. These measures show that diurnal changes occur in the relative sensitivity of the electroretino-graphic responses to light stimuli of different wavelength. Diurnal variations occur despite the absence of rhythmic photoperiodic stimulation. It is suggested that a spontaneous shift occurs in the functional dominance of cones and rods with a period near to 24 hours.