The transition from cone to rod vision was studied by measuring the variation in absolute threshold across the retina with a small monochromatic test stimulus (lambda = 490 nm). The shift from one mechanism to the other was determined by means of directional sensitivity. In the retinal region corresponding to rod vision, the test sensitivity (reciprocal threshold) decreased closer to the fovea. This variation may be attributed to three causes: a) the reduction in the number of rods; b) the increase in absorption of the yellow macular pigment; c) changes in the characteristics of the rods themselves or in the nervous conductors, which carry the signal to the brain, resulting in an increased perceptual threshold. The great importance of the latter cause is discussed. The threshold vs. intensity curves of cone vision showed two arms that may be attributed to the action of cone mechanisms. The existence of these two does not agree with what is expected from the simple trichromatic analysis of the results.