The transmission of neuronal impulses in the central visual system in kitten 7-42 days old has been investigated using electrophysiologic techniques. It was found that shortly after birth the axonal conduction velocities in the geniculocortical, in the corticogeniculate and in the commissural pathways of areas 17, 18, and 19 are less than 1.2 m/s, increasing up to maximal 10 m/s at the end of the first postnatal month. The monosynaptic delays across geniculocortical and commissural synapses in area 17 as measured during the third and fourth postnatal week are extremely long (between 2.8 and 6.1 ms) and do not change significantly during this time. During the first postnatal month, visual cortical neurons fatigue rapidly to visual and electrical stimulation; also, the proportion of neurons receiving polysynaptic inputs is much smaller than in the adult animal. Accordingly, neurons having complex receptive fields are rarely found in area 17 during the first postnatal month. Our experiments show that the geniculocortical and also the intracortical transmission of neuronal impulses are still immature at the end of the first postnatal month; at this time the systems for orientation and direction selectivity in the visual cortex are almost fully developed.