The number, affinity, and laminar distributions of various receptors in cat visual cortex were examined during postnatal development using homogenate and in vitro autoradiographic techniques. For all receptor populations examined, the total number of receptors (Bmax) increased from relatively low early values to peak values during the first three months of postnatal life followed by a drop or plateau in the number of receptors. This peak in Bmax occurred during the physiologically-defined period for cortical plasticity. For most receptors examined, the affinity (KD) was also altered during postnatal development. Many of the receptor populations examined exhibited changes in their initial laminar distributions during the first three months of postnatal development, although other did not. The results show a more complex picture of receptor ontogenesis than previously reported, and suggest that the observed receptor modifications affect the synaptic efficacy and the basic chemical circuitry of the visual cortex during the critical period.