ACTIVATION of NMDA (N-methyl-D-aspartate) receptors has recently been proposed as a prerequisite for the induction of experience-dependent modification of visual cortical neurones seen during early postnatal development. A new monoclonal antibody to the NMDA receptor subunit 1 (NMDA-R1) has been used to localize and compare the distribution of the receptors in the primary visual cortex of normal cats and those raised with either amblyopia induced by monocular optical blur or monocular esotropic strabismus. Although all three groups showed densest labelling in layers II-III, a comparison of immunopositive cells at any depth below the cortical surface showed a significantly lower frequency in strabismic and anisometropic cats than in normal cats, but a greater frequency in anisometropic cats that in strabismic animals. There appears to be no direct relationship between the expression of NMDA-R1 receptors and the level of excitability, binocularity or neuronal acuity known to exist in either of these two cat models of amblyopia, thus raising further questions as to the precise nature of the role of NMDA receptors in the processes of visual cortical plasticity.