We have studied glutamate receptor levels during very early phases of cortical formation by using quantitative in vitro autoradiography to map the expression of NMDA, AMPA and kainate receptors in the developing primary visual cortex of the ferret. NMDA and non-NMDA receptors exhibit very different developmental profiles in primary visual cortex. NMDA receptor density is low at birth and increases throughout the first 2 postnatal months, rising between threefold (layers II/III) and ninefold (layer VI). In contrast, AMPA receptors are abundant at birth and their density remains constant for the first postnatal month, before rising by a maximum of 1.7-fold (layer I) at around the time of eye-opening (postnatal day 32). Kainate receptors are also present in high levels at birth and their expression levels rise in the early postnatal period by between 1. 5-fold (layer I) and threefold (layers V/VI) to a peak just after eye-opening. The proportion of the total ionotropic glutamate receptor binding contributed by NMDA receptors thus rises from 5% at birth to a maximum of 22% at 2 months of age, while the AMPA receptor contribution falls from 87% to 72% over the same period. Below cortex, all three glutamate receptor subtypes are expressed in the subplate region for the first 3 postnatal weeks. These developmental patterns, combined with the fact that AMPA receptors are densely expressed in the proliferative zones underlying presumptive area 17, indicate that non-NMDA receptor expression levels in primary visual cortex are mostly specified much earlier than those of NMDA receptors.