We studied 587 cells in the striate cortex of 47 kittens, ranging in age postnatally from the 8th day to the 20th week, to explore differences in rates of functional maturation of cells and afferents among cortical layers. For all cells studied we determined spontaneous activity level, visual responsiveness, responsiveness to electrical stimulation of the afferent pathways and histologically reconstructed their laminar localization. At the 2nd-3rd week postnatally, single shocks applied to the dorsal lateral geniculate nucleus (LGN) elicited relatively inconsistent responses with long latencies in most of the cells in all layers, except in layer V where responses were consistent. In layers IVab, V, and VIu (the upper half of layer VI) the LGN-latency decreased very rapidly in the 4th week, while in layer IVc such a decrease occurred in the 5th week. In layers II+III and VIl (the lower half of layer VI) a less substantial decrease in latency occurred during the 4th and 5th week. At 2 weeks, nearly half of layer V cells had orientation-specific visual responses or spontaneous activity, but such cells were rare in other layers. The proportions of the specific or spontaneously active cells matured successively in the order of layers V leads to IVab and VIu leads to IVc leads to II+III and VIl. We conclude that the maturation of functional properties of cortical cells may occur successively in the above-mentioned laminar order, and that cells size and connectivity with afferents may be factors influencing the rate of functional maturation of cortical cells. The intracortical synaptic delay was estimated for each of the cells driven mono-synaptically from the LGN. The average delay decreased most rapidly during the 4th and 5th week. Conduction velocities of afferents innervating the mono-synaptic cells in layers IVab and IVc were calculated. The velocity of the former afferents increased very quickly and reached a value suggesting myelination at 4 weeks, while that of the latter afferents at 5 weeks. Since each type of LGN cell is known to project separately to layers IVab and IVc, respectively, this suggests that Y-cells of LGN may mature earlier than X-cells.